[NVPTX] Handle __nvvm_reflect("__CUDA_ARCH").

Summary:
libdevice in recent CUDA versions relies on __nvvm_reflect() to select
GPU-specific bitcode. This patch addresses the requirement.

Reviewers: jlebar

Subscribers: jholewinski, sanjoy, hiraditya, bixia, llvm-commits

Differential Revision: https://reviews.llvm.org/D50207

llvm-svn: 338908