[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: #58858
Differential Revision: https://reviews.llvm.org/D138049

openmp/runtime/src/exports_so.txt

@@ -72,10 +72,8 @@ VERSION {
         __kmp_fork_call;
         __kmp_invoke_microtask;
 #if KMP_USE_MONITOR
-        __kmp_launch_monitor;
         __kmp_reap_monitor;
 #endif
-        __kmp_launch_worker;
         __kmp_reap_worker;
         __kmp_release_64;
         __kmp_wait_64;

openmp/runtime/src/kmp_runtime.cpp

@@ -9195,3 +9195,20 @@ void __kmp_set_nesting_mode_threads() {
   if (__kmp_nesting_mode == 1) // turn on nesting for this case only
     set__max_active_levels(thread, __kmp_nesting_mode_nlevels);
 }
+
+// Empty symbols to export (see exports_so.txt) when feature is disabled
+extern "C" {
+#if !KMP_STATS_ENABLED
+void __kmp_reset_stats() {}
+#endif
+#if !USE_DEBUGGER
+int __kmp_omp_debug_struct_info = FALSE;
+int __kmp_debugging = FALSE;
+#endif
+#if !USE_ITT_BUILD || !USE_ITT_NOTIFY
+void __kmp_itt_fini_ittlib() {}
+void __kmp_itt_init_ittlib() {}
+#endif
+}
+
+// end of file

openmp/runtime/src/z_Linux_util.cpp

@@ -979,7 +979,7 @@ void __kmp_exit_thread(int exit_status) {
 #if KMP_USE_MONITOR
 void __kmp_resume_monitor();
 
-void __kmp_reap_monitor(kmp_info_t *th) {
+extern "C" void __kmp_reap_monitor(kmp_info_t *th) {
   int status;
   void *exit_val;
 
@@ -1021,6 +1021,12 @@ void __kmp_reap_monitor(kmp_info_t *th) {
 
   KMP_MB(); /* Flush all pending memory write invalidates.  */
 }
+#else
+// Empty symbol to export (see exports_so.txt) when
+// monitor thread feature is disabled
+extern "C" void __kmp_reap_monitor(kmp_info_t *th) {
+  (void)th;
+}
 #endif // KMP_USE_MONITOR
 
 void __kmp_reap_worker(kmp_info_t *th) {