x86-cleanups-2022-12-10

  - Rework the handling of x86_regset for 32 and 64 bit. The original
    implementation tried to minimize the allocation size with quite some
    hard to understand and fragile tricks. Make it robust and straight
    forward by separating the register enumerations for 32 and 64 bit
    completely.

  - Add a few missing static annotations

  - Remove the stale unused setup_once() assembly function

  - Address a few minor static analysis and kernel-doc warnings