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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175283 91177308-0d34-0410-b5e6-96231b3b80d8