tsan: re-enable mmap_stress.cpp test

[dvyukov]

Summary

The comment says it was flaky in 2016,
but it wasn't possible to debug it back then.
Re-enable the test at least on linux/x86_64.
It will either work, or at least we should
see failure output from lit today.

compiler-rt/test/tsan/mmap_stress.cpp

 // RUN: %clang_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
+// This test is flaky on several builders:
+// https://groups.google.com/d/msg/llvm-dev/KUFPdLhBN3Q/L75rwW9xBgAJ
+// The cause is unknown (lit hides test output on failures).
+// Let's try to re-enable it at least in one configutaion.
+// REQUIRES: linux, x86_64-target-arch
 #include "test.h"
 #include <errno.h>
 #include <sys/mman.h>
 
 void *SubWorker(void *arg) {
   (void)arg;
   const int kMmapSize =  65536;
   for (int i = 0; i < 500; i++) {
void *Worker(void *arg) {
   for (int i = 0; i < 4; i++) {
     if (pthread_join(th[i], 0))
       exit(printf("pthread_join failed: %d\n", errno));
   }
   return 0;
 }
 
 int main() {
-  // This test is flaky on several builders:
-  // https://groups.google.com/d/msg/llvm-dev/KUFPdLhBN3Q/L75rwW9xBgAJ
-  // The cause is unknown (lit hides test output on failures).
-#if 0
   pthread_t th[4];
   for (int i = 0; i < 4; i++) {
     if (pthread_create(&th[i], 0, Worker, 0))
       exit(printf("pthread_create failed: %d\n", errno));
   }
   for (int i = 0; i < 4; i++) {
     if (pthread_join(th[i], 0))
       exit(printf("pthread_join failed: %d\n", errno));
   }
-#endif
   fprintf(stderr, "DONE\n");
 }
 
 // CHECK: DONE