Currently, RETPOLINE*_CFLAGS are defined in the top-level Makefile
but only x86 makes use of them. Move them there. If ever another
architecture finds the need, it can be reconsidered.

  [ bp: Massage a bit. ]
Suggested-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NKees Cook <keescook@chromium.org>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20211119165630.219152765@infradead.org