Rollup merge of rust-lang#48195 - paoloteti:compiler-builtins-update,…

… r=alexcrichton

Update compiler-builtins to latest master.

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