[compiler-rt] Adding -lm to the clang driver when using compiler-rt

Hey all,
I have a small change to add -lm when using compiler-rt in https://reviews.llvm.org/D49330, since it appears that some of the builtins need it. I got linker errors from __divdc3, __divsc3, and __divxc3, but there could others.

It seems like a simple change to me and works for the cases I’ve tried, but before landing it I thought I’d bring it up here to see if anyone has concerns about it. Feel free to add yourself as a reviewer if you have any opinion about this :slight_smile:

Thanks – Jordan