[tsan] Fix stack_sync_reuse.cc test on OS X

[kubamracek]

Summary

The test relies on two variables in different frames to end up being on the same address. For some reason, this isn't true on OS X. This patch adds __attribute__((aligned(64))) to the variables, which actually makes the variables occupy the same address. This is still not a guarantee, but it's more likely to work (the test looks very fragile already).

test/tsan/stack_sync_reuse.cc

while (__atomic_load_n(&syncp, __ATOMIC_ACQUIRE) == 0)
     usleep(1000);  // spin wait
   global = 42;
   __atomic_store_n(syncp, 1, __ATOMIC_RELEASE);
   __atomic_store_n(&syncp, 0, __ATOMIC_RELAXED);
   return NULL;
 }
 
 void __attribute__((noinline)) foobar() {
-  long s;
+  __attribute__((aligned(64))) long s;
+
   addr = &s;
   __atomic_store_n(&s, 0, __ATOMIC_RELAXED);
   __atomic_store_n(&syncp, &s, __ATOMIC_RELEASE);
   while (__atomic_load_n(&syncp, __ATOMIC_RELAXED) != 0)
     usleep(1000);  // spin wait
 }
 
 void __attribute__((noinline)) barfoo() {
-  long s;
+  __attribute__((aligned(64))) long s;
+
   if (addr != &s) {
     printf("address mismatch addr=%p &s=%p\n", addr, &s);
     exit(1);
   }
   __atomic_store_n(&addr, &s, __ATOMIC_RELAXED);
   __atomic_store_n(&s, 0, __ATOMIC_RELAXED);
   sink = __atomic_load_n(&s, __ATOMIC_ACQUIRE);
   global = 43;