OvmfPkg/XenBusDxe: Add InterlockedCompareExchange16.

OvmfPkg/XenBusDxe: Add InterlockedCompareExchange16.

This patch is inspired by InterlockedCompareExchange32 from the
BaseSynchronizationLib.

The function will be used in the "OvmfPkg/XenBusDxe: Add Grant Table
functions" patch.

Change in V3:
- Implement both .S and .asm, to get rid of GCC specific asm.
- Implement 32bit part of the assembly

Change in V2:
- Add intel compilation code
  MSFT code is not compied over because I don't know how it works.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16263 6f19259b-4bc3-4df7-8a09-765794883524