[tests] Specify the dependence to NVPTX backend for Polly ACC test cases

Some Polly ACC test cases fail without a working NVPTX backend. We explicitly
specify this dependence in REQUIRES. Alternatively, we could have only marked
polly-acc as supported in case the NVPTX backend is available, but as we might
use other backends in the future, this does not seem to be the best choice.

For this to work, we also need to make the 'targets_to_build' information
available.

Suggested-by: Michael Kruse <llvm@meinersbur.de>
llvm-svn: 296853