arm64: Create a KBI for managing CPU ID registers

[andrew]

Summary

Create a new group of functions to manage a view of the ID registers
that may be different than both userspace and the kernel views.

This will be used by bhyve to provide the sanitized register values.

Sponsored by: Arm Ltd

[markj]

sys/arm64/arm64/identcpu.c

1803

This function name doesn't make much sense to me. The function doesn't update anything, it clamps individual fields in a register value (new_val) so that they are no larger than the existing fields (in val).

Maybe this should be mask_fields() or clamp_fields() or something like that? Or am I missing some ARM terminology?

1895

How do the register fields initially get populated?

sys/arm64/include/cpu.h

222

There is no cpu_desc_set().