Do not run GPGPU test cases without nvptx target

Tag the GPGPU codegen test cases as unsupported if the nvptx target is not
included in the current llvm build.

Contributed-by:  Yabin Hu <yabin.hwu@gmail.com>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@208779 91177308-0d34-0410-b5e6-96231b3b80d8