Add print headers

Can't do printing until % and / operators are implemented.
Link GCC builtins

Doing this just for div and mod implementations.
Add itoa partial implementation

Currently doesn't completely pay attention to buffer size. Also added
digits() function into math.h
init.c: Minimalistic main loop

init.c file is still modified for testing and such. This makes testing a bit
tricky but is probably the best solution for now.