librustc: This commit add a linker of sorts. This allows us to by-pass the
toolchain altogether (& all those blasted bugs that arise from LLVM version
differences) and emit stable pexe's directly.
Sadly, linking still takes its time; for the most part we adopt what the
toolchain does which is to pull everything into a huge module and then run all
IR -> PNaCl IR passes on it, though we do do limited runs on our crates beforehand.