arm64: export new HWCAP features

Expose some of the new HWCAP features added in r65304. This includes the
addition of elf_hwcap2 into the sysvec, and a separate function to parse
for those features.

This only exposes features which require no further configuration, e.g.
indicating the presence of certain instructions. Larger features (SVE)
will not be advertised until we actually support them. The exact list of
features/extensions this patch exposes is:

• ARMv8.0-DGH
• ARMv8.0-SB
• ARMv8.2-BF16
• ARMv8.2-DCCVADP
• ARMv8.2-I8MM
• ARMv8.4-LRCPC
• ARMv8.5-CondM
• ARMv8.5-FRINT
• ARMv8.5-RNG
• PSTATE.SSBS

While here, annotate elf_hwcap and elf_hwcap2 as __read_frequently, and
move the declarations to the machine/md_var.h header.

Submitted by: mikael@ (D22314 portion)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26031
Differential Revision: https://reviews.freebsd.org/D22314