[ASan, LSan] Improve tracking of thread creation.

In the current scheme of things, the call to ThreadStart() in the child
thread is not synchronized with the parent thread. So, if a pointer is passed to
pthread_create, there may be a window of time during which this pointer will not
be discoverable by LSan. I.e. the pthread_create interceptor has already
returneed and thus the pointer is no longer on the parent stack, but we don't
yet know the location of the child stack. This has caused bogus leak reports
(see http://llvm.org/bugs/show_bug.cgi?id=21621/).

This patch makes the pthread_create interceptor wait until the child thread is
properly registered before returning.