[OpenMP][FIX] Ensure __kmpc_kernel_parallel is reachable

The problem is we create the call to __kmpc_kernel_parallel in the
openmp-opt pass but while we optimize the code, the call is not there
yet. Thus, we assume we never reach it from __kmpc_target_deinit. That
allows us to remove the store in there (`ParallelRegionFn = nullptr`),
which leads to bad results later on.

This is a shortstop solution until we come up with something better.

Fixes https://github.com/llvm/llvm-project/issues/57064