64-bit signed/unsigned overflow checked multiplication unavailable on 32-bit

[thestinger]

LLVM relies on the existence of support in compiler-rt for this on some platforms, but we're using libgcc so it's broken.

http://llvm.org/bugs/show_bug.cgi?id=14469

I suspect we are going to run into this again as we gain support for more of LLVM's capabilities, for now I'm just going to disable CheckedMul for i64 and u64 on 32-bit.

[alexcrichton]

This is fixable now that we are linking against compiler-rt