UefiCpuPkg: error: invalid instruction mnemonic	'retf'

.S assembler files must be AT&T syntax ones. So Intel syntax mnemonic is not good. Discovered by clang integrated assembler.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Nikolai Saoukh <nms@otdel-1.org>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/UefiCpuPkg@16100 6f19259b-4bc3-4df7-8a09-765794883524