Get rid of globals.asm. Use ::s to export labels between objects.

globals.asm was a hack that:
- gave rgbasm an extra 100k lines to read
- increased reliance on the python preprocessor
- made the makefile a mess
- gave object creation an extra step
- wasn't even necessary

This speeds up build time by at least a third.
Clean up makefile messages and prerequisite handling
Split wram.asm from poke{red,blue}.o into wram.o

wram.asm still defines some constants. They're moved into constants/wram_constants.asm.
Makefile: condense dependency scanning into one loop