Mark variables referenced from assembler files visible.

This fixes compile problems with LTO.

Cc: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1391845930-28580-4-git-send-email-ak@linux.intel.comSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>