selftests/x86/apx: Add APX test

The extended general-purpose registers for APX may contain random data,
which is currently assumed by the xstate testing framework. This allows
the testing of the new userspace feature using the common test code.

Invoke the test entry function from apx.c after enumerating the
state component and adding it to the support list

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250416021720.12305-6-chang.seok.bae@intel.com