This adds the basic support required for debugging with gdb over remote serial protocol.
Includes register read/write for general purpose and status registers, but not floating point registers at this time.
The GDB register numbers are defined here: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/arch/aarch64.h;h=b75352461a412e74aa8e397507ff76a1824de1c7;hb=HEAD