MIPS: Fallback CPU -march flag to ISA level if unsupported

LLVM does not implement some of -march options. However those options
are not mandatory for kernel to build for those CPUs.

Fallback -march CFLAG to ISA level if unsupported by toolchain so
we can get those kernel to build with LLVM.

Link: https://github.com/ClangBuiltLinux/linux/issues/1544Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/Makefile

@@ -148,10 +148,10 @@ cflags-y += $(call cc-option,-Wa$(comma)-mno-fix-loongson3-llsc,)
 #
 # CPU-dependent compiler/assembler options for optimization.
 #
-cflags-$(CONFIG_CPU_R3000)	+= -march=r3000
-cflags-$(CONFIG_CPU_R4300)	+= -march=r4300 -Wa,--trap
-cflags-$(CONFIG_CPU_R4X00)	+= -march=r4600 -Wa,--trap
-cflags-$(CONFIG_CPU_TX49XX)	+= -march=r4600 -Wa,--trap
+cflags-$(CONFIG_CPU_R3000)	+= $(call cc-option,-march=r3000,-march=mips1)
+cflags-$(CONFIG_CPU_R4300)	+= $(call cc-option,-march=r4300,-march=mips3) -Wa,--trap
+cflags-$(CONFIG_CPU_R4X00)	+= $(call cc-option,-march=r4600,-march=mips3) -Wa,--trap
+cflags-$(CONFIG_CPU_TX49XX)	+= $(call cc-option,-march=r4600,-march=mips3) -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS32_R1)	+= -march=mips32 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS32_R2)	+= -march=mips32r2 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS32_R5)	+= -march=mips32r5 -Wa,--trap -modd-spreg
@@ -160,26 +160,30 @@ cflags-$(CONFIG_CPU_MIPS64_R1)	+= -march=mips64 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS64_R2)	+= -march=mips64r2 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS64_R5)	+= -march=mips64r5 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS64_R6)	+= -march=mips64r6 -Wa,--trap
-cflags-$(CONFIG_CPU_P5600)	+= -march=p5600 -Wa,--trap -modd-spreg
-cflags-$(CONFIG_CPU_R5000)	+= -march=r5000 -Wa,--trap
-cflags-$(CONFIG_CPU_R5500)	+= $(call cc-option,-march=r5500,-march=r5000) \
+cflags-$(CONFIG_CPU_P5600)	+= $(call cc-option,-march=p5600,-march=mips32r5) \
+			-Wa,--trap -modd-spreg
+cflags-$(CONFIG_CPU_R5000)	+= $(call cc-option,-march=r5000,-march=mips4) \
 			-Wa,--trap
-cflags-$(CONFIG_CPU_NEVADA)	+= $(call cc-option,-march=rm5200,-march=r5000) \
+cflags-$(CONFIG_CPU_R5500)	+= $(call cc-option,-march=r5500,-march=mips4) \
 			-Wa,--trap
-cflags-$(CONFIG_CPU_RM7000)	+= $(call cc-option,-march=rm7000,-march=r5000) \
+cflags-$(CONFIG_CPU_NEVADA)	+= $(call cc-option,-march=rm5200,-march=mips4) \
 			-Wa,--trap
-cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-march=sb1,-march=r5000) \
+cflags-$(CONFIG_CPU_RM7000)	+= $(call cc-option,-march=rm7000,-march=mips4) \
+			-Wa,--trap
+cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-march=sb1,-march=mips64) \
 			-Wa,--trap
 cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-mno-mdmx)
 cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-mno-mips3d)
-cflags-$(CONFIG_CPU_R10000)	+= $(call cc-option,-march=r10000,-march=r8000) \
+cflags-$(CONFIG_CPU_R10000)	+= $(call cc-option,-march=r10000,-march=mips4) \
 			-Wa,--trap
 cflags-$(CONFIG_CPU_CAVIUM_OCTEON) += -march=octeon -Wa,--trap
 cflags-$(CONFIG_CAVIUM_CN63XXP1) += -Wa,-mfix-cn63xxp1
 cflags-$(CONFIG_CPU_BMIPS)	+= -march=mips32 -Wa,-mips32 -Wa,--trap
 
-cflags-$(CONFIG_CPU_LOONGSON2E) += -march=loongson2e -Wa,--trap
-cflags-$(CONFIG_CPU_LOONGSON2F) += -march=loongson2f -Wa,--trap
+cflags-$(CONFIG_CPU_LOONGSON2E) += \
+			$(call cc-option,-march=loongson2e,-march=mips3) -Wa,--trap
+cflags-$(CONFIG_CPU_LOONGSON2F) += \
+			$(call cc-option,-march=loongson2f,-march=mips3) -Wa,--trap
 # Some -march= flags enable MMI instructions, and GCC complains about that
 # support being enabled alongside -msoft-float. Thus explicitly disable MMI.
 cflags-$(CONFIG_CPU_LOONGSON2EF) += $(call cc-option,-mno-loongson-mmi)