Each distinct XSAVE feature has a number assigned to it.  Among other
things, the number determines the ordering of features in the XSAVE
buffer and is also used to generate XSAVE bitmasks like the value
for XCR0.

AMX state is dynamically enabled by the architecture-specific prctl().
This prctl() takes one XSAVE feature number as an argument.  However, the
feature numbers are not defined in any readily available userspace headers.
The means that each userspace app trying to use dynamic feature prctl()s
will likely end up defining their own constants for each feature.

Since these feature numbers are a part of the uabi, expose them in the
prctl() uabi header.  Save everyone the trouble of looking them up and
defining their own.

[ dhansen: expand changelog a bit ]

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/all/20230121001900.14900-3-chang.seok.bae%40intel.com