UefiCpuPkg: Avoid "error A2085" when DDK3790 tool chain is used

UefiCpuPkg: Avoid "error A2085" when DDK3790 tool chain is used

The DDK3790 tool chain fails when the PAUSE instruction is assembled:
error A2085: instruction or register not accepted in current CPU mode The solution is to use the .686 directive along with the .xmm directive.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Scott Duplichan <scott@notabs.org>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17134 6f19259b-4bc3-4df7-8a09-765794883524