Update compiler-builtins to latest master.

- Rebase compiler-rt to LLVM 6
- New VFP intrinsics on ARM
- Add generic conversion from a narrower to a wider FP type (f32->f64)
- Fixes minor issues on _subsf3, __subdf3 and __aeabi_fcmple
- Split test suite to a separate crate

Author: paoloteti