arm64: vdso32: require CROSS_COMPILE_COMPAT for gcc+bfd

Similar to
commit 231ad7f4 ("Makefile: infer --target from ARCH for CC=clang")
There really is no point in setting --target based on
$CROSS_COMPILE_COMPAT for clang when the integrated assembler is being
used, since
commit ef943405 ("arm64: vdso32: drop -no-integrated-as flag").

Allows COMPAT_VDSO to be selected without setting $CROSS_COMPILE_COMPAT
when using clang and lld together.

Before:
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make -j72 LLVM=1 defconfig
$ grep CONFIG_COMPAT_VDSO .config
CONFIG_COMPAT_VDSO=y
$ ARCH=arm64 make -j72 LLVM=1 defconfig
$ grep CONFIG_COMPAT_VDSO .config
$

After:
$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make -j72 LLVM=1 defconfig
$ grep CONFIG_COMPAT_VDSO .config
CONFIG_COMPAT_VDSO=y
$ ARCH=arm64 make -j72 LLVM=1 defconfig
$ grep CONFIG_COMPAT_VDSO .config
CONFIG_COMPAT_VDSO=y
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20211019223646.1146945-5-ndesaulniers@google.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -1264,7 +1264,8 @@ config KUSER_HELPERS
 
 config COMPAT_VDSO
 	bool "Enable vDSO for 32-bit applications"
-	depends on !CPU_BIG_ENDIAN && "$(CROSS_COMPILE_COMPAT)" != ""
+	depends on !CPU_BIG_ENDIAN
+	depends on (CC_IS_CLANG && LD_IS_LLD) || "$(CROSS_COMPILE_COMPAT)" != ""
 	select GENERIC_COMPAT_VDSO
 	default y
 	help

arch/arm64/kernel/vdso32/Makefile

@@ -10,18 +10,15 @@ include $(srctree)/lib/vdso/Makefile
 
 # Same as cc-*option, but using CC_COMPAT instead of CC
 ifeq ($(CONFIG_CC_IS_CLANG), y)
-CC_COMPAT_CLANG_FLAGS := --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
-
 CC_COMPAT ?= $(CC)
-CC_COMPAT += $(CC_COMPAT_CLANG_FLAGS)
-
-ifneq ($(LLVM),)
-LD_COMPAT ?= $(LD)
+CC_COMPAT += --target=arm-linux-gnueabi
 else
-LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
+CC_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
 endif
+
+ifeq ($(CONFIG_LD_IS_LLD), y)
+LD_COMPAT ?= $(LD)
 else
-CC_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
 LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
 endif
 
@@ -45,10 +42,6 @@ VDSO_CPPFLAGS += $(LINUXINCLUDE)
 # Common C and assembly flags
 # From top-level Makefile
 VDSO_CAFLAGS := $(VDSO_CPPFLAGS)
-ifneq ($(shell $(CC_COMPAT) --version 2>&1 | head -n 1 | grep clang),)
-VDSO_CAFLAGS += --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
-endif
-
 VDSO_CAFLAGS += $(call cc32-option,-fno-PIE)
 ifdef CONFIG_DEBUG_INFO
 VDSO_CAFLAGS += -g