[asan/lsan] Avoid possible deadlock in dynamic ASan runtime thread initialization.

There is possible deadlock in dynamic ASan runtime when we dlopen() shared lib
which creates a thread at the global initialization stage. The scenario:

1) dlopen grabs a GI_pthread_mutex_lock in main thread.
2) main thread calls pthread_create, ASan intercepts it, calls real pthread_create
   and waits for the second thread to be "fully initialized".
3) Newly created thread tries to access a thread local disable_counter in LSan
   (to complete its "full initialization") and hangs in tls_get_addr_tail, because
   it also tries to acquire GI_pthread_mutex_lock.

The issue is reproducible on relative recent Glibc versions e.g. 2.23.

Differential Revision: https://reviews.llvm.org/D26028

llvm-svn: 285385