x86, alternative: Add altinstruction_entry macro

Add altinstruction_entry macro to generate .altinstructions section
entries from assembly code.  This should be less failure-prone than
open-coding.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1305671358-14478-5-git-send-email-fenghua.yu@intel.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>