x86, alternative: Use .pushsection/.popsection

.section/.previous doesn't nest.  Use .pushsection/.popsection in
<asm/alternative.h> so that they can be properly nested.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348256595-29119-5-git-send-email-hpa@linux.intel.com