Add NT_ARM_ADDR_MASK

This can be used by debuggers to find which bits in a virtual address
should be masked off to get a canonical address. This is currently used
by the Pointer Authentication Code support to get its mask. It could also
be used if we support Top Byte Ignore for the same purpose.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34302