[libomptarget][test] Add support for APU testing feature. (#82054)

Add test and support for `// REQUIRES: apu` for the category of tests
which exercise APU specific behavior.

Note: when running on an actual APU you may have to use the following if
the architecture ID is not enough to determine if the underlying device
is an APU:

```
IS_APU=1 ninja check-openmp
```