x86, mwait: Move mwait constants to a common header file

We have MWAIT constants spread across three different .c files, for no
good reason.  Move them all into a common header file.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Reviewed-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Len Brown <lenb@kernel.org>
LKML-Reference: <tip-*@git.kernel.org>