Add aarch64 support to GDB.

This includes support for cross-debugging aarch64 process cores and
kernels as well as native support for aarch64 processes. Hardware
single stepping is supported, but hardware watchpoints are not yet
supported.

Reviewed by: luca.pizzamiglio@gmail.com (maintainer)
Approved by: az (implicit)
Differential Revision: https://reviews.freebsd.org/D12074