[sanitizer] Start background thread once

[morehouse]

LGTM

compiler-rt/lib/sanitizer_common/sanitizer_common_libcdep.cpp

void SetSoftRssLimitExceededCallback(void (*Callback)(bool exceeded)) {
   SoftRssLimitExceededCallback = Callback;
 }
 
 #if (SANITIZER_LINUX || SANITIZER_NETBSD) && !SANITIZER_GO
 // Weak default implementation for when sanitizer_stackdepot is not linked in.
 SANITIZER_WEAK_ATTRIBUTE StackDepotStats StackDepotGetStats() { return {}; }
 
 void *BackgroundThread(void *arg) {
+  VPrintf(1, "%s: Started BackgroundThread\n", SanitizerToolName);
   const uptr hard_rss_limit_mb = common_flags()->hard_rss_limit_mb;
   const uptr soft_rss_limit_mb = common_flags()->soft_rss_limit_mb;
   const bool heap_profile = common_flags()->heap_profile;
   uptr prev_reported_rss = 0;
   uptr prev_reported_stack_depot_size = 0;
   bool reached_soft_rss_limit = false;
   uptr rss_during_last_reported_profile = 0;
   while (true) {

 
 void MaybeStartBackgroudThread() {
 #if (SANITIZER_LINUX || SANITIZER_NETBSD) && \
     !SANITIZER_GO  // Need to implement/test on other platforms.
   // Start the background thread if one of the rss limits is given.
   if (!common_flags()->hard_rss_limit_mb &&
       !common_flags()->soft_rss_limit_mb &&
       !common_flags()->heap_profile) return;
-  if (!&real_pthread_create) return;  // Can't spawn the thread anyway.
+  if (!&real_pthread_create) {
+    VPrintf(1, "%s: real_pthread_create undefined\n", SanitizerToolName);
+    return;  // Can't spawn the thread anyway.
+  }
+
+  static bool started = false;
+  if (!started) {
+    started = true;
     internal_start_thread(BackgroundThread, nullptr);
+  }
 #endif
 }
 
 static void (*sandboxing_callback)();
 void SetSandboxingCallback(void (*f)()) {
   sandboxing_callback = f;
 }