Populate the GBA ROM header so that emulators and physical Game Boy
Advance consoles recognize the ROM as a valid game.
Note: The reserve space at the end of the header was hand-tuned. Why
this magic value?
Thanks to Kyle Lemons for the inspiration and original design. The
implementation in this commit is very different however, building on top
of the software vectoring needed in RISC-V. The result is a flexible
interrupt handler that does not take up any RAM for configuration.
