[compiler-rt][asan] Fix for flaky asan check

This fixes #87324.

We haven't been able to come up with a minimal reproducer but we can
reliabely avoid this failure with the following fix. Prior to the
GetGlobalLowLevelAllocator change, the old LowLevelAllocator aquired a
lock associated with it preventing that specific allocator from being
accessed at the same time by many threads. With the
GetGlobalLowLevelAllocator change, I had accidentally replaced it but
not taken into account the lock, so we can have a data race if the
allocator is used at any point while a thread is being created. The
global allocator can be used for flag parsing or registering asan
globals.

Author: PiJoules

compiler-rt/lib/asan/asan_thread.cpp

@@ -44,10 +44,11 @@ static ThreadRegistry *asan_thread_registry;
 static ThreadArgRetval *thread_data;
 
 static Mutex mu_for_thread_context;
+static LowLevelAllocator allocator_for_thread_context;
 
 static ThreadContextBase *GetAsanThreadContext(u32 tid) {
   Lock lock(&mu_for_thread_context);
-  return new (GetGlobalLowLevelAllocator()) AsanThreadContext(tid);
+  return new (allocator_for_thread_context) AsanThreadContext(tid);
 }
 
 static void InitThreads() {