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

LLVM revision 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
450e7dd4 ("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.

Reviewed-by: Nick Desaulneris <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191119045712.39633-4-natechancellor@gmail.com