arm,arm64: Add a NT_ARM_TLS read-only register set.

This register set exposes the per-thread TLS register. It matches the
layout used by Linux on arm64. Linux does not implement this note for
32-bit arm.

Reviewed by: andrew, markj
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34595