arm64: Support passing more registers to signals

To support recent extensions to the Arm architecture we may need to
store more or larger registers when sending a signal.

To support this create a list of these extra registers. Userspace that
needs to access a register in the signal handler can then walk the list
to find the correct register struct and read/write its contents.

Reviewed by: kib, markj (earlier version)
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43302