arm64: Add a masked get_kernel_reg()

This lets consumers fetch the value of a system register and apply a
mask over individual fields. That is, each field in the returned value
will be the "smaller" of the two provided by "mask" and the value saved
in kern_cpu_desc. This will be used by vmm to sanitize host system
register fields.

Reviewed by: andrew
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc. (hardware)
Differential Revision: https://reviews.freebsd.org/D40500