Use a regset for NT_ARM_VFP.

This includes adding support for NT_ARM_VFP for 32-bit binaries
running under aarch64 kernels both for ptrace(), and coredumps via the
kernel and gcore.

Reviewed by: andrew, markj
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34448

(cherry picked from commit add00c381e5c9938133061c401dcba764009cafc)