x86/asm changes for v6.8:

 - Replace magic numbers in GDT descriptor definitions & handling:

   - Introduce symbolic names via macros for descriptor types/fields/flags,
     and then use these symbolic names.

   - Clean up definitions a bit, such as GDT_ENTRY_INIT()

   - Fix/clean up details that became visibly inconsistent after the
     symbol-based code was introduced:

      - Unify accessed flag handling

      - Set the D/B size flag consistently & according to the HW specification

Signed-off-by: Ingo Molnar <mingo@kernel.org>