runtime: fix false race report during tracing

runtime: fix false race report during tracing

Currently race detector produces the following reports on pprof tests:

WARNING: DATA RACE
Read by goroutine 4:
  runtime/pprof_test.TestTraceStartStop()
      src/runtime/pprof/trace_test.go:38 +0x1da
  testing.tRunner()
      src/testing/testing.go:448 +0x13a

Previous write by goroutine 5:
  bytes.(*Buffer).grow()
      src/bytes/buffer.go:102 +0x190
  bytes.(*Buffer).Write()
      src/bytes/buffer.go:127 +0x75
  runtime/pprof.func·002()
      src/runtime/pprof/pprof.go:633 +0xae

Trace writer goroutine synchronizes with StopTrace
using trace.shutdownSema runtime semaphore.
But race detector does not see that synchronization
and so produces false reports.
Teach race detector about the synchronization.

Change-Id: I1219817325d4e16b423f29a0cbee94c929793881
Reviewed-on: https://go-review.googlesource.com/3746
Reviewed-by: Russ Cox <rsc@golang.org>