selftests/x86/avx: Add AVX tests

Add xstate testing specifically for those vector register states,
validating kernel's context switching and ensuring ABI compliance.
Use the established xstate testing framework.

Alternatively, this invocation could be placed directly in
xstate.c::main(). However, the current test file naming convention, which
clearly specifies the tested area, seems reasonable. Adding avx.c
considerably aligns with that convention.

The test output should be like this for ZMM_Hi256 as an example:

  $ avx_64
  ...
  [RUN]   AVX-512 ZMM_Hi256: check context switches, 10 iterations, 5 threads.
  [OK]    No incorrect case was found.
  [RUN]   AVX-512 ZMM_Hi256: inject xstate via ptrace().
  [OK]    'xfeatures' in SW reserved area was correctly written
  [OK]    xstate was correctly updated.
  [RUN]   AVX-512 ZMM_Hi256: 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]   AVX-512 ZMM_Hi256: load new xstate from sighandler and check it after sigreturn
  [OK]    xstate was restored correctly

But systems without AVX-512 will look like:
  ...
  The kernel does not support feature number: 5
  The kernel does not support feature number: 6
  The kernel does not support feature number: 7

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/20250226010731.2456-10-chang.seok.bae@intel.com

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