[CUDA] Add -fcuda-approx-transcendentals flag.

Summary:
This lets us emit e.g. sin.approx.f32.  See
http://docs.nvidia.com/cuda/parallel-thread-execution/#floating-point-instructions-sin

Reviewers: rnk

Subscribers: tra, cfe-commits

Differential Revision: http://reviews.llvm.org/D20493

llvm-svn: 270484