selftests/x86/xstate: Refactor XSAVE helpers for general use

The AMX test introduced several XSAVE-related helper functions, but so
far, it has been the only user of them. These helpers can be generalized
for broader test of multiple xstate features.

Move most XSAVE-related code into xsave.h, making it shareable. The
restructuring includes:

 * Establishing low-level XSAVE helpers for saving and restoring register
  states, as well as handling XSAVE buffers.

 * Generalizing state data manipuldations: set_rand_data()

 * Introducing a generic feature query helper: get_xstate_info()

While doing so, remove unused defines in amx.c.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250226010731.2456-3-chang.seok.bae@intel.com