ARM: pass -march= only to compiler

When both -march= and -Wa,-march= are specified for assembler or
assembler-with-cpp sources, GCC and Clang will prefer the -Wa,-march=
value but Clang will warn that -march= is unused.

warning: argument unused during compilation: '-march=armv6k'
[-Wunused-command-line-argument]

This is the top group of warnings we observe when using clang to
assemble the kernel via `ARCH=arm make LLVM=1`.

Split the arch-y make variable into two, so that -march= flags only get
passed to the compiler, not the assembler. -D flags are added to
KBUILD_CPPFLAGS which is used for both C and assembler-with-cpp sources.

Clang is trying to warn that it doesn't support different values for
-march= and -Wa,-march= (like GCC does, but the kernel doesn't need this)
though the value of the preprocessor define __thumb2__ is based on
-march=. Make sure to re-set __thumb2__ via -D flag for assembler
sources now that we're no longer passing -march= to the assembler. Set
it to a different value than the preprocessor would for -march= in case
-march= gets accidentally re-added to KBUILD_AFLAGS in the future.
Thanks to Ard and Nathan for this suggestion.

Link: ClangBuiltLinux#1315
Link: ClangBuiltLinux#1587
Link: llvm/llvm-project#55656
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

arch/arm/Makefile

@@ -57,21 +57,34 @@ endif
 KBUILD_CFLAGS	+= $(call cc-option,-fno-ipa-sra)
 
 # This selects which instruction set is used.
+arch-$(CONFIG_CPU_32v7M)	:=-march=armv7-m
+arch-$(CONFIG_CPU_32v7)		:=-march=armv7-a
+arch-$(CONFIG_CPU_32v6)		:=-march=armv6
+# Only override the compiler option if ARMv6. The ARMv6K extensions are
+# always available in ARMv7
+ifeq ($(CONFIG_CPU_32v6),y)
+arch-$(CONFIG_CPU_32v6K)	:=-march=armv6k
+endif
+arch-$(CONFIG_CPU_32v5)		:=-march=armv5te
+arch-$(CONFIG_CPU_32v4T)	:=-march=armv4t
+arch-$(CONFIG_CPU_32v4)		:=-march=armv4
+arch-$(CONFIG_CPU_32v3)		:=-march=armv3m
+
 # Note that GCC does not numerically define an architecture version
 # macro, but instead defines a whole series of macros which makes
 # testing for a specific architecture or later rather impossible.
-arch-$(CONFIG_CPU_32v7M)	:=-D__LINUX_ARM_ARCH__=7 -march=armv7-m
-arch-$(CONFIG_CPU_32v7)		:=-D__LINUX_ARM_ARCH__=7 -march=armv7-a
-arch-$(CONFIG_CPU_32v6)		:=-D__LINUX_ARM_ARCH__=6 -march=armv6
-# Only override the compiler opt:ion if ARMv6. The ARMv6K extensions are
+cpp-$(CONFIG_CPU_32v7M)		:=-D__LINUX_ARM_ARCH__=7
+cpp-$(CONFIG_CPU_32v7)		:=-D__LINUX_ARM_ARCH__=7
+cpp-$(CONFIG_CPU_32v6)		:=-D__LINUX_ARM_ARCH__=6
+# Only override the compiler option if ARMv6. The ARMv6K extensions are
 # always available in ARMv7
 ifeq ($(CONFIG_CPU_32v6),y)
-arch-$(CONFIG_CPU_32v6K)	:=-D__LINUX_ARM_ARCH__=6 -march=armv6k
+cpp-$(CONFIG_CPU_32v6K)		:=-D__LINUX_ARM_ARCH__=6
 endif
-arch-$(CONFIG_CPU_32v5)		:=-D__LINUX_ARM_ARCH__=5 -march=armv5te
-arch-$(CONFIG_CPU_32v4T)	:=-D__LINUX_ARM_ARCH__=4 -march=armv4t
-arch-$(CONFIG_CPU_32v4)		:=-D__LINUX_ARM_ARCH__=4 -march=armv4
-arch-$(CONFIG_CPU_32v3)		:=-D__LINUX_ARM_ARCH__=3 -march=armv3m
+cpp-$(CONFIG_CPU_32v5)		:=-D__LINUX_ARM_ARCH__=5
+cpp-$(CONFIG_CPU_32v4T)		:=-D__LINUX_ARM_ARCH__=4
+cpp-$(CONFIG_CPU_32v4)		:=-D__LINUX_ARM_ARCH__=4
+cpp-$(CONFIG_CPU_32v3)		:=-D__LINUX_ARM_ARCH__=3
 
 # This selects how we optimise for the processor.
 tune-$(CONFIG_CPU_ARM7TDMI)	:=-mtune=arm7tdmi
@@ -116,15 +129,16 @@ AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
 
 ifeq ($(CONFIG_THUMB2_KERNEL),y)
 CFLAGS_ISA	:=-mthumb -Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
-AFLAGS_ISA	:=$(CFLAGS_ISA) -Wa$(comma)-mthumb
+AFLAGS_ISA	:=$(CFLAGS_ISA) -Wa$(comma)-mthumb -D__thumb2__=2
 else
 CFLAGS_ISA	:=$(call cc-option,-marm,) $(AFLAGS_NOWARN)
 AFLAGS_ISA	:=$(CFLAGS_ISA)
 endif
 
 # Need -Uarm for gcc < 3.x
+KBUILD_CPPFLAGS	+=$(cpp-y)
 KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
-KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y) -include asm/unified.h -msoft-float
+KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) -Wa,$(arch-y) $(tune-y) -include asm/unified.h -msoft-float
 
 CHECKFLAGS	+= -D__arm__