runtime/race: fix memory leak

The leak was reported internally on a sever canary that runs for days.
After a day server consumes 5.6GB, after 6 days -- 12.2GB.
The leak is exposed by the added benchmark.
The leak is fixed upstream in :
http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/tsan/rtl/tsan_rtl_thread.cc?view=diff&r1=276102&r2=276103&pathrev=276103

Fixes #16441

Change-Id: I9d4f0adef48ca6cf2cd781b9a6990ad4661ba49b
Reviewed-on: https://go-review.googlesource.com/25091
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>

src/runtime/race/README

@@ -4,4 +4,4 @@ the LLVM project (http://llvm.org/git/compiler-rt.git).
 
 To update the .syso files use golang.org/x/build/cmd/racebuild.
 
-Current runtime is built on rev 9d79ea3416bfbe3acac50e47802ee9621bf53254.
+Current runtime is built on rev e35e7c00b5c7e7ee5e24d537b80cb0d34cebb038.

src/runtime/race/race_test.go

@@ -221,3 +221,21 @@ func BenchmarkSyncLeak(b *testing.B) {
 	}
 	wg.Wait()
 }
+
+func BenchmarkStackLeak(b *testing.B) {
+	done := make(chan bool, 1)
+	for i := 0; i < b.N; i++ {
+		go func() {
+			growStack(rand.Intn(100))
+			done <- true
+		}()
+		<-done
+	}
+}
+
+func growStack(i int) {
+	if i == 0 {
+		return
+	}
+	growStack(i - 1)
+}