Integrate UB fix in floatsidf and floatsisf

[japaric]

cf. rust-lang/compiler-rt#24

These fixes are not in the llvm-mirror/compiler-rt repo which we are using.

[alexcrichton]

Ideally we'd upstream, but that may take a bit...

Agreed should use a fork in the meantime to speed things along.

[japaric]

So, we could just switch to the rust-lang/compiler-rt repo.

[alexcrichton]

👍