llvm-gcc-toolfile: remove -frounding-math for Clang

`-frounding-math` is not supported in Clang, thus remove it.
Tested with LLVM/Clang 3.5.0
```
clang: warning: optimization flag '-frounding-math' is not supported
[-Winvalid-command-line-argument]
```
Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>