OvmfPkg/XenBusDxe: Add TestAndClearBit.

OvmfPkg/XenBusDxe: Add TestAndClearBit.

This atomically test's and clear's a bit.

Change in V3:
- adding IA32 support. (not yet reviewed)
  both XenBusDxe/Ia32/TestAndClearBit.{S,asm} are new

Change in V2:
- Adding .asm version
- Comment the function

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16266 6f19259b-4bc3-4df7-8a09-765794883524