gdbstub: Update registers and sizes for aarch64

This gets gdbstub working at least to the point where clients can
communicate with it.

What works:
- Reading/writing GPRegs
- Reading/writing memory
- Interrupting the emulated program and continuing

What does NOT work:
- Breakpoints. Sizes have been updated to u64, but support will need to be
  added in the interpreter for them to work.
- VRegs. Mostly because my gdb was having issues with 128-bit regs for
  some reason. However, the current u128 representation is a bit
  awkward to use and should probably be updated first.