[OpenMP][libomp] Cleanup version script and exported symbols

This patch fixes issues seen once https://reviews.llvm.org/D135402 is applied.

The exports_so.txt file attempts to export functions which may not exist
depending on which features are enabled/disabled in the OpenMP
runtime library. There are not many of these so exporting dummy
symbols is feasible.

* Export dummy __kmp_reset_stats() function when stats is disabled.
* Export dummy debugging data when USE_DEBUGGER is disabled
* Export dummy __kmp_itt_[fini|init]_ittlib() functions
   when ITT Notify is disabled
* Export dummy __kmp_reap_monitor() function when KMP_USE_MONITOR
  is disabled
* Remove __kmp_launch_monitor and __kmp_launch_worker from being exported.
  They have been static symbols since library inception.

Fixes: https://github.com/llvm/llvm-project/issues/58858
Differential Revision: https://reviews.llvm.org/D138049