[GPGPU] Add support for NVIDIA libdevice

Summary:
This allows us to map functions such as exp, expf, expl, for which no
LLVM intrinsics exist. Instead, we link to NVIDIA's libdevice which provides
high-performance implementations of a wide range of (math) functions. We
currently link only a small subset, the exp, cos and copysign functions. Other
functions will be enabled as needed.

Reviewers: bollu, singam-sanjay

Reviewed By: bollu

Subscribers: tstellar, tra, nemanjai, pollydev, mgorny, llvm-commits, kbarton

Tags: #polly

Differential Revision: https://reviews.llvm.org/D35703

llvm-svn: 309560