arm64: extend struct db_reg to include watchpoint registers

The motivation is to provide access to these registers from userspace
via ptrace(2) requests PT_GETDBREGS and PT_SETDBREGS.

This change breaks the ABI of these particular requests, but is
justified by the fact that the intended consumers (debuggers) have not
been taught to use them yet. Making this change now enables active
upstream work on lldb to begin using this interface, and take advantage
of the hardware debugging registers available on the platform.

PR: 252860
Reported by: Michał Górny (mgorny@gentoo.org)
Reviewed by: andrew, 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/D28415