[Polly] Fix REQUIRES for nvptx-dependent tests

These have been effectively disabled ever since 'nvptx' was added to
the REQUIRES clauses, because REQUIRES does not support triple checks.
The new 'target=<triple>' is supported, so switch to that scheme.
Fix up XFAIL annotations, now that these tests are actually run.

Part of the project to eliminate special handling for triples in lit
expressions.

Differential Revision: https://reviews.llvm.org/D139728