selftests/x86/xstate: Introduce signal ABI test

With the refactored test cases, another xstate exposure to userspace is
through signal delivery. While amx.c includes signal-related scenarios,
its primary focus is on xstate permission management, which is largely
specific to dynamic states.

The remaining gap is testing xstate preservation and restoration across
signal delivery. The kernel defines an ABI for presenting xstate in the
signal frame, closely resembling the hardware XSAVE format, where xstate
modification is also possible.

Introduce a new test case to verify xstate preservation across signal
delivery and return, that is ensuring ABI compatibility by:

 - Loading xstate before raising a signal.

 - Verifying correct exposure in the signal frame

 - Modifying xstate in the signal frame before returning.

 - Checking the state restoration upon signal return.

Integrate this test into the AMX test suite as an initial usage site.

  Expected output:
  $ amx_64
  ...
  [RUN]   AMX Tile data: load xstate and raise SIGUSR1
  [OK]    'magic1' is valid
  [OK]    'xfeatures' in SW reserved area is valid
  [OK]    'xfeatures' in XSAVE header is valid
  [OK]    xstate delivery was successful
  [OK]    'magic2' is valid
  [RUN]   AMX Tile data: load new xstate from sighandler and check it after sigreturn
  [OK]    xstate was restored correctly

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/20250226010731.2456-7-chang.seok.bae@intel.com