x86, cmpxchg: Use __compiletime_error() to make usage messages a bit nicer

Use __compiletime_error() to produce a compile-time error rather than
link-time, where available.

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>