Add NetBSD/aarch64 gdbserver support

The support is on par with NetBSD/amd64, thus GPR works,
single step and software breakpoint are operational, and the
SVR4 r_debug integration is functional.

gdbserver/ChangeLog:

         * netbsd-aarch64-low.cc: Add.
         * Makefile.in (SFILES): Register "netbsd-aarch64-low.c".
         * configure.srv: Add aarch64*-*-netbsd*.