[OpenMP] Fix a test that fails when 'libgomp' is the default library

We can't do offloading with libgomp, thus the test fails. This change
explicitly chooses an OpenMP runtime library that is capable of
offloading.

This change is similar to
https://github.com/llvm/llvm-project/commit/a5098e5f27badc3ba16533418accd2e17641e4e4.