x86/selftests: Skip the tests if prerequisites aren't fulfilled

x86/selftests: Skip the tests if prerequisites aren't fulfilled

Skip instead of failing when prerequisite conditions aren't fulfilled,
such as invalid xstate values etc.

Make the tests show as 'SKIP' when run:

  make -C tools/testing/selftests/ TARGETS=x86 run_tests

  ...
  # timeout set to 45
  # selftests: x86: amx_64
  # # xstate cpuid: invalid tile data size/offset: 0/0
  ok 42 selftests: x86: amx_64 # SKIP
  # timeout set to 45
  # selftests: x86: lam_64
  # # Unsupported LAM feature!
  ok 43 selftests: x86: lam_64 # SKIP
  ...

In the AMX test, Move away from check_cpuid_xsave() and start using
arch_prctl() to find out if AMX support is present or not. In the
kernels where AMX isn't present, arch_prctl() returns -EINVAL, hence it is
backward compatible.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/r/20240327111720.3509180-1-usama.anjum@collabora.com