x86, asm: Cleanup thunk_64.S

Drop thunk_ra macro in favor of an additional argument to the thunk
macro since their bodies are almost identical. Do a whitespace scrubbing
and use CFI-aware macros for full annotation.

Signed-off-by: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/1306873314-32523-5-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>