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>
---

Index: UefiCpuPkg/CpuDxe/Ia32/MpAsm.asm
===================================================================
--- UefiCpuPkg/CpuDxe/Ia32/MpAsm.asm	(revision 17108)
+++ UefiCpuPkg/CpuDxe/Ia32/MpAsm.asm	(working copy)
@@ -11,7 +11,8 @@
 ;
 ;------------------------------------------------------------------------------
 
-.586p
+.686
+.xmm
 .model flat, C
 
 extern mTopOfApCommonStack:DWORD