Fix racy msan test

[maxim-kuvyrkov]

Summary

Child thread may finish before pthread_setname_np() and/or
pthread_getname_np() had a chance to run, which causes these functions
to fail with ENOENT (meaning /proc/self/task/[tid]/comm doesn't exist).

Fix by stalling child thread on a mutex.

compiler-rt/trunk/test/msan/pthread_getname_np.cc

 // RUN: %clangxx_msan -std=c++11 -O0 %s -o %t && %run %t
 // UNSUPPORTED: android, netbsd
 
 // Regression test for a deadlock in pthread_getattr_np
 
 #include <assert.h>
 #include <pthread.h>
 #include <string.h>
 #include <sanitizer/msan_interface.h>
 
 #include <stdio.h>
 
+// Stall child thread on this lock to make sure it doesn't finish
+// before the end of the pthread_getname_np() / pthread_setname_np() tests.
+static pthread_mutex_t lock;
+
 void *ThreadFn(void *) {
+  pthread_mutex_lock (&lock);
+  pthread_mutex_unlock (&lock);
   return nullptr;
 }
 
 int main(void) {
   pthread_t t;
+
+  pthread_mutex_init (&lock, NULL);
+  pthread_mutex_lock (&lock);
+
   int res = pthread_create(&t, 0, ThreadFn, 0);
   assert(!res);
 
   const char *kMyThreadName = "my-thread-name";
   res = pthread_setname_np(t, kMyThreadName);
   assert(!res);
 
   char buf[100];
   res = pthread_getname_np(t, buf, sizeof(buf));
   assert(!res);
   assert(strcmp(buf, kMyThreadName) == 0);
 
+  pthread_mutex_unlock (&lock);
+
   res = pthread_join(t, 0);
   assert(!res);
   return 0;
 }