powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

r374662 gives LLVM the ability to convert certain loops into a reference
to bcmp as an optimization; this breaks prom_init_check.sh:

  CALL    arch/powerpc/kernel/prom_init_check.sh
Error: External symbol 'bcmp' referenced from prom_init.c
make[2]: *** [arch/powerpc/kernel/Makefile:196: prom_init_check] Error 1

bcmp is defined in lib/string.c as a wrapper for memcmp so this could be
added to the whitelist. However, commit 450e7dd ("powerpc/prom_init:
don't use string functions from lib/") copied memcmp as prom_memcmp to
avoid KASAN instrumentation so having bcmp be resolved to regular memcmp
would break that assumption. Furthermore, because the compiler is the
one that inserted bcmp, we cannot provide something like prom_bcmp.

To prevent LLVM from being clever with optimizations like this, use
-ffreestanding to tell LLVM we are not hosted so it is not free to make
transformations like this.

Link: ClangBuiltLinux#647
Link: llvm/llvm-project@76cdcf2
Reviewed-by: Nick Desaulneris <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

arch/powerpc/kernel/Makefile

@@ -21,7 +21,7 @@ CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 
-CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
+CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector) -ffreestanding
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code