GB: Fix GBC games running on GBA with BIOS
GB: Fix GBC games running on GBA with BIOS

GBC games can detect if they are running on a GBA by checking if the
low bit of register B is set to 1.  B is set in `GBSkipBIOS`, but
B was not updated when loading a real BIOS.  Load 1 into B after
unmapping the BIOS if the model is set to GBA.

Iniquitatis/mgba,Anty-Lemon/mgba,mgba-emu/mgba,libretro/mgba,Anty-Lemon/mgba,mgba-emu/mgba,mgba-emu/mgba,Iniquitatis/mgba,libretro/mgba,libretro/mgba,libretro/mgba,Iniquitatis/mgba,libretro/mgba,mgba-emu/mgba,Iniquitatis/mgba,Anty-Lemon/mgba,Anty-Lemon/mgba