This patch also introduces testing for libomptarget-nvptx
which has been missing until now. I propose to add tests for
all bugs that are fixed in the future.
The target check-libomptarget-nvptx is not run by default because
- we can't determine if there is a GPU plugged into the system.
- it will require the latest Clang compiler. Keeping compatibility with older releases would prevent testing newer code generation developed in trunk.