x86/fpu/xstate: Introduce xfeature order table and accessor macro

x86/fpu/xstate: Introduce xfeature order table and accessor macro

The kernel has largely assumed that higher xstate component numbers
correspond to later offsets in the buffer. However, this assumption no
longer holds for the non-compacted format, where a newer state component
may have a lower offset.

When iterating over xstate components in offset order, using the feature
number as an index may be misleading. At the same time, the CPU exposes
each component’s size and offset based on its feature number, making it a
key for state information.

To provide flexibility in handling xstate ordering, introduce a mapping
table: feature order -> feature number.  The table is dynamically
populated based on the CPU-exposed features and is sorted in offset order
at boot time.

Additionally, add an accessor macro to facilitate sequential traversal of
xstate components based on their actual buffer positions, given a feature
bitmask. This accessor macro will be particularly useful for computing
custom non-compacted format sizes and iterating over xstate offsets in
non-compacted buffers.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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/20250320234301.8342-3-chang.seok.bae@intel.com