UefiCpuPkg CpuExceptionHandlerLib: Use %rax instead of %eax to make code consistence.

Make code consistence between ASM and S files.

(Sync patch r16530 from main trunk.)

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/branches/UDK2014.SP1/UefiCpuPkg@16533 6f19259b-4bc3-4df7-8a09-765794883524