[OpenMP][libomptarget] Separate lit tests for different offloading targets (1/2)

This patch creates a separate test directory for each offloading target to be
tested. This allows to test multiple architectures in one configuration, while
still see all failing tests separately. The lit test names include the target
triple, so that it will be easier to spot the failing target.

This patch also allows to mark expected failing tests based on the
target-triple, as the currently used triple is added to the lit "features":
```
// XFAIL: nvptx64-nvidia-cuda
```

Differential Revision: https://reviews.llvm.org/D101315