Add target features for sm_* and ptx*, both of which form a partial
order, but cannot be combined to a single partial order. These mirror
the LLVM target features, but we do not provide LLVM target
processors (which imply both an sm_* and ptx* feature).
Add some documentation for the nvptx target.
Combining revisions with only-arch allows specifying
that a test only applies to a handful of targets.
This allows removing a large amount of repetition
in the test suite for tests that do not benefit.
The revisions are suboptimal for this for some tests,
so they aren't preferred in those cases.
