arm64: handle watchpoint exceptions from EL0

This is a prerequisite to allowing the use of hardware watchpoints for
userspace debuggers.

This is also a slight departure from the x86 behaviour, since si_addr
returns the data address that triggered the watchpoint, not the
address of the instruction that was executed. Otherwise, there is no
straightforward way for the application to determine which watchpoint
was triggered. Make a note of this in the siginfo(3) man page.

Reviewed by: jhb, markj (earlier version)
Tested by: Michał Górny (mgorny@gentoo.org)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28561