add a flag to prevent unnecessary push/pop

[camthesaxman]

A number of GBA games (such as Legend of Zelda: A Link to the Past, Pokemon Pinball: Ruby and Sapphire, and Advance Wars) seem to use the newer agbcc compiler, but have a slight when it comes to saving lr. agbcc sometimes pushes lr onto the stack in leaf functions where it isn't necessary, while old_agbcc does not. Using the -fprologue-bugfix flag will revert agbcc to the behavior of old_agbcc in this case.

[rawr51919]

Please rebase so a rebuild can occur with the latest CI stuff present in the current agbcc repo