[OpenMP/Offload] Enable Offload runtime and add check

[jplehr]

This enables the new "offload" runtime that
llvm/llvm-project#75125 will create. In addition, it adds a new lit test target.
I removed the extra CMake argument that would again specify which runtimes to build manually, so that we rely on the factory to add the respective runtimes.

Since the PR does not make check-libomptarget available in the top level configuration, we have to change the ninja working directory into the runtimes build folder, where the target is known. Whether or not this works with this crude "additional lit checks" mechanism is an experiment.

[jplehr]

A couple of more comments:

• This affects: https://lab.llvm.org/staging/#/builders/185
• I think we should wait for @saiislam to let us know of the status on his downstreaming efforts of the llvm-project PR before proceeding with it and use this buildbot change as a test in the meantime to make sure that the add_lit_check works as intended.
• This will make the bot red if it works. The message should be that it cded into the directory, but the target name is unknown. If the bot prints a different error, adding this target via the add_lit_checks liekly did not work.

[jplehr]

I just realized that I'll have to change the patch to test the add_lit_checks mechanism. As it is now, the bot will bail out early, given that offload is unknown.

[jplehr]

I created #142 which should only add the additional lit target. Meaning that we can use it to see if the machinery is able to handle what we need.

[jplehr]

I'll close this PR, since we discussed a different route during a meeting today.