Further isolate kernel from userspace, prevent accidental inclusion of
undesireable headers, mainly float.h and stdatomic.h.

nds32 keeps -isystem globally due to intrinsics used in entrenched header.

-isystem is selectively reenabled for some files, again, for intrinsics.

Compile tested on:

hexagon-defconfig hexagon-allmodconfig
alpha-allmodconfig alpha-allnoconfig alpha-defconfig arm64-allmodconfig
arm64-allnoconfig arm64-defconfig arm-am200epdkit arm-aspeed_g4
arm-aspeed_g5 arm-assabet arm-at91_dt arm-axm55xx arm-badge4 arm-bcm2835
arm-cerfcube arm-clps711x arm-cm_x300 arm-cns3420vb arm-colibri_pxa270
arm-colibri_pxa300 arm-collie arm-corgi arm-davinci_all arm-dove
arm-ep93xx arm-eseries_pxa arm-exynos arm-ezx arm-footbridge arm-gemini
arm-h3600 arm-h5000 arm-hackkit arm-hisi arm-imote2 arm-imx_v4_v5
arm-imx_v6_v7 arm-integrator arm-iop32x arm-ixp4xx arm-jornada720
arm-keystone arm-lart arm-lpc18xx arm-lpc32xx arm-lpd270 arm-lubbock
arm-magician arm-mainstone arm-milbeaut_m10v arm-mini2440 arm-mmp2
arm-moxart arm-mps2 arm-multi_v4t arm-multi_v5 arm-multi_v7 arm-mv78xx0
arm-mvebu_v5 arm-mvebu_v7 arm-mxs arm-neponset arm-netwinder arm-nhk8815
arm-omap1 arm-omap2plus arm-orion5x arm-oxnas_v6 arm-palmz72 arm-pcm027
arm-pleb arm-pxa arm-pxa168 arm-pxa255-idp arm-pxa3xx arm-pxa910
arm-qcom arm-realview arm-rpc arm-s3c2410 arm-s3c6400 arm-s5pv210
arm-sama5 arm-shannon arm-shmobile arm-simpad arm-socfpga arm-spear13xx
arm-spear3xx arm-spear6xx arm-spitz arm-stm32 arm-sunxi arm-tct_hammer
arm-tegra arm-trizeps4 arm-u8500 arm-versatile arm-vexpress arm-vf610m4
arm-viper arm-vt8500_v6_v7 arm-xcep arm-zeus csky-allmodconfig
csky-allnoconfig csky-defconfig h8300-edosk2674 h8300-h8300h-sim
h8300-h8s-sim i386-allmodconfig i386-allnoconfig i386-defconfig
ia64-allmodconfig ia64-allnoconfig ia64-bigsur ia64-generic ia64-gensparse
ia64-tiger ia64-zx1 m68k-amcore m68k-amiga m68k-apollo m68k-atari
m68k-bvme6000 m68k-hp300 m68k-m5208evb m68k-m5249evb m68k-m5272c3
m68k-m5275evb m68k-m5307c3 m68k-m5407c3 m68k-m5475evb m68k-mac
m68k-multi m68k-mvme147 m68k-mvme16x m68k-q40 m68k-stmark2 m68k-sun3
m68k-sun3x microblaze-allmodconfig microblaze-allnoconfig microblaze-mmu
mips-ar7 mips-ath25 mips-ath79 mips-bcm47xx mips-bcm63xx mips-bigsur
mips-bmips_be mips-bmips_stb mips-capcella mips-cavium_octeon mips-ci20
mips-cobalt mips-cu1000-neo mips-cu1830-neo mips-db1xxx mips-decstation
mips-decstation_64 mips-decstation_r4k mips-e55 mips-fuloong2e
mips-gcw0 mips-generic mips-gpr mips-ip22 mips-ip27 mips-ip28 mips-ip32
mips-jazz mips-jmr3927 mips-lemote2f mips-loongson1b mips-loongson1c
mips-loongson2k mips-loongson3 mips-malta mips-maltaaprp mips-malta_kvm
mips-malta_qemu_32r6 mips-maltasmvp mips-maltasmvp_eva mips-maltaup
mips-maltaup_xpa mips-mpc30x mips-mtx1 mips-nlm_xlp mips-nlm_xlr
mips-omega2p mips-pic32mzda mips-pistachio mips-qi_lb60 mips-rb532
mips-rbtx49xx mips-rm200 mips-rs90 mips-rt305x mips-sb1250_swarm
mips-tb0219 mips-tb0226 mips-tb0287 mips-vocore2 mips-workpad mips-xway
nds32-allmodconfig nds32-allnoconfig nds32-defconfig nios2-10m50
nios2-3c120 nios2-allmodconfig nios2-allnoconfig openrisc-allmodconfig
openrisc-allnoconfig openrisc-or1klitex openrisc-or1ksim
openrisc-simple_smp parisc-allnoconfig parisc-generic-32bit
parisc-generic-64bit powerpc-acadia powerpc-adder875 powerpc-akebono
powerpc-amigaone powerpc-arches powerpc-asp8347 powerpc-bamboo
powerpc-bluestone powerpc-canyonlands powerpc-cell powerpc-chrp32
powerpc-cm5200 powerpc-currituck powerpc-ebony powerpc-eiger
powerpc-ep8248e powerpc-ep88xc powerpc-fsp2 powerpc-g5 powerpc-gamecube
powerpc-ge_imp3a powerpc-holly powerpc-icon powerpc-iss476-smp
powerpc-katmai powerpc-kilauea powerpc-klondike powerpc-kmeter1
powerpc-ksi8560 powerpc-linkstation powerpc-lite5200b powerpc-makalu
powerpc-maple powerpc-mgcoge powerpc-microwatt powerpc-motionpro
powerpc-mpc512x powerpc-mpc5200 powerpc-mpc7448_hpc2 powerpc-mpc8272_ads
powerpc-mpc8313_rdb powerpc-mpc8315_rdb powerpc-mpc832x_mds
powerpc-mpc832x_rdb powerpc-mpc834x_itx powerpc-mpc834x_itxgp
powerpc-mpc834x_mds powerpc-mpc836x_mds powerpc-mpc836x_rdk
powerpc-mpc837x_mds powerpc-mpc837x_rdb powerpc-mpc83xx
powerpc-mpc8540_ads powerpc-mpc8560_ads powerpc-mpc85xx_cds
powerpc-mpc866_ads powerpc-mpc885_ads powerpc-mvme5100 powerpc-obs600
powerpc-pasemi powerpc-pcm030 powerpc-pmac32 powerpc-powernv
powerpc-ppa8548 powerpc-ppc40x powerpc-ppc44x powerpc-ppc64
powerpc-ppc64e powerpc-ppc6xx powerpc-pq2fads powerpc-ps3
powerpc-pseries powerpc-rainier powerpc-redwood powerpc-sam440ep
powerpc-sbc8548 powerpc-sequoia powerpc-skiroot powerpc-socrates
powerpc-storcenter powerpc-stx_gp3 powerpc-taishan powerpc-tqm5200
powerpc-tqm8540 powerpc-tqm8541 powerpc-tqm8548 powerpc-tqm8555
powerpc-tqm8560 powerpc-tqm8xx powerpc-walnut powerpc-warp powerpc-wii
powerpc-xes_mpc85xx riscv-allmodconfig riscv-allnoconfig riscv-nommu_k210
riscv-nommu_k210_sdcard riscv-nommu_virt riscv-rv32 s390-allmodconfig
s390-allnoconfig s390-debug s390-zfcpdump sh-ap325rxa sh-apsh4a3a
sh-apsh4ad0a sh-dreamcast sh-ecovec24 sh-ecovec24-romimage sh-edosk7705
sh-edosk7760 sh-espt sh-hp6xx sh-j2 sh-kfr2r09 sh-kfr2r09-romimage
sh-landisk sh-lboxre2 sh-magicpanelr2 sh-microdev sh-migor sh-polaris
sh-r7780mp sh-r7785rp sh-rsk7201 sh-rsk7203 sh-rsk7264 sh-rsk7269
sh-rts7751r2d1 sh-rts7751r2dplus sh-sdk7780 sh-sdk7786 sh-se7206 sh-se7343
sh-se7619 sh-se7705 sh-se7712 sh-se7721 sh-se7722 sh-se7724 sh-se7750
sh-se7751 sh-se7780 sh-secureedge5410 sh-sh03 sh-sh2007 sh-sh7710voipgw
sh-sh7724_generic sh-sh7757lcr sh-sh7763rdp sh-sh7770_generic sh-sh7785lcr
sh-sh7785lcr_32bit sh-shmin sh-shx3 sh-titan sh-ul2 sh-urquell
sparc-allmodconfig sparc-allnoconfig sparc-sparc32 sparc-sparc64
um-i386-allmodconfig um-i386-allnoconfig um-i386-defconfig
um-x86_64-allmodconfig um-x86_64-allnoconfig x86_64-allmodconfig
x86_64-allnoconfig x86_64-defconfig xtensa-allmodconfig xtensa-allnoconfig
xtensa-audio_kc705 xtensa-cadence_csp xtensa-common xtensa-generic_kc705
xtensa-iss xtensa-nommu_kc705 xtensa-smp_lx200 xtensa-virt
xtensa-xip_kc705

Tested-by: Nathan Chancellor <nathan@kernel.org> # build (hexagon)
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>