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>
Merge pull request #1171 from davidlt/remove-clang-frounding-math

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