More assembly optimisations.
video.c, video.h, cpu.h: Move to the cross-platform directory.

bios.c: Remove all high-level emulation functions.

common.h: Add cross-platform function declarations ReGBA_RenderScreen and ReGBA_IsRenderingNextFrame, as well as variable declarations GBAScreen and GBAScreenPitch to be filled by the ports. Use most of them elsewhere.