[Libomptarget] Add a test for the `libc` implementation of assert (#69518)

Summary:
The `libcgpu.a` file provides its own implementation of `__assert_fail`.
This adds a test to make sure it's usable in OpenMP offloading as
expected. Currently this requires linking `libcgpu.a` before the OpenMP
device RTL however. We also disable the test on the CPU as the format of
the string will be different.