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
(cherry picked from commit b2cb74c22c4f7087f342cf50b116b040de6bdc6c)