ARM: Reduce __thumb2__ definition to crypto files that require it

Commit 1d2e9b6 ("ARM: 9265/1: pass -march= only to compiler") added
a __thumb2__ define to ASFLAGS to avoid build errors in the crypto code,
which relies on __thumb2__ for preprocessing. Commit 59e2cf8 ("ARM:
9275/1: Drop '-mthumb' from AFLAGS_ISA") followed up on this by removing
-mthumb from AFLAGS so that __thumb2__ would not be defined when the
default target was ARMv7 or newer.

Unfortunately, the second commit's fix assumes that the toolchain
defaults to -mno-thumb / -marm, which is not the case for Debian's
arm-linux-gnueabihf target, which defaults to -mthumb:

  $ echo | arm-linux-gnueabihf-gcc -dM -E - | grep __thumb
  #define __thumb2__ 1
  #define __thumb__ 1

This target is used by several CI systems, which will still see
redefined macro warnings, despite '-mthumb' not being present in the
flags:

  <command-line>: warning: "__thumb2__" redefined
  <built-in>: note: this is the location of the previous definition

Remove the global AFLAGS __thumb2__ define and move it to the crypto
folder where it is required by the imported OpenSSL algorithms; the rest
of the kernel should use the internal CONFIG_THUMB2_KERNEL symbol to
know whether or not Thumb2 is being used or not. Be sure that __thumb2__
is undefined first so that there are no macro redefinition warnings.

Link: ClangBuiltLinux#1772
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

arch/arm/Makefile

@@ -132,7 +132,7 @@ AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
 
 ifeq ($(CONFIG_THUMB2_KERNEL),y)
 CFLAGS_ISA	:=-Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
-AFLAGS_ISA	:=$(CFLAGS_ISA) -Wa$(comma)-mthumb -D__thumb2__=2
+AFLAGS_ISA	:=$(CFLAGS_ISA) -Wa$(comma)-mthumb
 CFLAGS_ISA	+=-mthumb
 else
 CFLAGS_ISA	:=$(call cc-option,-marm,) $(AFLAGS_NOWARN)

arch/arm/crypto/Makefile

@@ -53,7 +53,12 @@ $(obj)/%-core.S: $(src)/%-armv4.pl
 
 clean-files += poly1305-core.S sha256-core.S sha512-core.S
 
+aflags-thumb2-$(CONFIG_THUMB2_KERNEL)  := -U__thumb2__ -D__thumb2__=1
+
+AFLAGS_sha256-core.o += $(aflags-thumb2-y)
+AFLAGS_sha512-core.o += $(aflags-thumb2-y)
+
 # massage the perlasm code a bit so we only get the NEON routine if we need it
 poly1305-aflags-$(CONFIG_CPU_V7) := -U__LINUX_ARM_ARCH__ -D__LINUX_ARM_ARCH__=5
 poly1305-aflags-$(CONFIG_KERNEL_MODE_NEON) := -U__LINUX_ARM_ARCH__ -D__LINUX_ARM_ARCH__=7
-AFLAGS_poly1305-core.o += $(poly1305-aflags-y)
+AFLAGS_poly1305-core.o += $(poly1305-aflags-y) $(aflags-thumb2-y)