enable SDISel sincos optimization for GNU environments

- add sincos to runtime library if target triple environment is GNU
- added canCombineSinCosLibcall() which checks that sincos is in the RTL and
  if the environment is GNU then unsafe fpmath is enabled (required to
  preserve errno)
- extended sincos-opt lit test

Reviewed by: Hal Finkel

llvm-svn: 175283