Add a Makefile.rules, that defines basic rules to build and link
GCW0: Set the program name to "regba"
GCW0: Allow building an OPK by typing "make opk"
GCW Zero: Compile with -mno-abicalls -mno-shared to avoid using global relocations and register $28.
Merge https://github.com/pcercuei/ReGBA
GCW Zero: -mno-shared is not actually needed. Add the new name of the executable to .gitignore.