x86: Add xadd helper macro

Add a common xadd implementation.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Link: http://lkml.kernel.org/r/4E5BCC40.3030501@goop.org


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>