selftests/x86/xstate: Enumerate and name xstate components

After moving essential helpers from amx.c, the code remains neutral
regarding which xstate components it handles. However, explicitly listing
known components helps users identify which features are ready for
testing.

Enumerate xstate components to facilitate identification. Extend struct
xstate_info to include a name field, providing a human-readable
identifier.

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/20250226010731.2456-4-chang.seok.bae@intel.com