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>