Store core dump notes for all valid register sets for FreeBSD processes.

In particular, use a generic wrapper around struct regset rather than
requiring per-regset helpers. This helper replaces the MI
elfN(note_prstatus) and elfN(note_fpregset) helpers. It also
removes the need to explicitly dump NT_ARM_ADDR_MASK in the arm64
__elfN(dump_thread).

Reviewed by: markj, emaste
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34446