[OpenMP] Fix lone target exit data

Without this patch, an `omp target exit data` before the runtime is
initialized produces a runtime error.  This patch fixes that by
changing `__tgt_target_data_end_mapper` to call `CheckDeviceAndCtors`
like many other runtime routines.

Discussed at
<https://lists.llvm.org/pipermail/openmp-dev/2021-March/003920.html>.

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D97907