x86, xsave: Check cpuid level for XSTATE_CPUID (0x0d)

The patch introduces the XSTATE_CPUID macro and adds a check that
tests if XSTATE_CPUID exists.

Signed-off-by: Robert Richter <robert.richter@amd.com>
LKML-Reference: <1279731838-1522-4-git-send-email-robert.richter@amd.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>