[MLton] bytecode and Makefile

[Matthew Fluet]

I propose moving /bytecode to /runtime/bytecode.

Mainly, I'm motivated by the fact that /runtime/Makefile has accumulated 
the logic for compiling C code under different targets and gcc versions; 
I'd rather not duplicate the logic, where it becomes harder to maintain. 
Secondly, /Makefile immediately archives bytecode/interpret.o into 
libmlton.a, so logically, it would appear to belong in /runtime.