runtime/race: TestOutput flake on linux-amd64-race

[mdempsky]

runtime/race.TestOutput flaked on the linux-amd64-race trybot for golang.org/cl/37102: https://storage.googleapis.com/go-build-log/a6236839/linux-amd64-race_6bbdcdcb.log

I don't see any similar issues previously reported.

--- FAIL: TestOutput (25.32s)
	output_test.go:66: failed test case external_cgo_thread, expect:
		==================
		WARNING: DATA RACE
		Read at 0x[0-9,a-f]+ by main goroutine:
		  main\.main\(\)
		      .*/main\.go:34 \+0x[0-9,a-f]+
		
		Previous write at 0x[0-9,a-f]+ by goroutine [0-9]:
		  main\.goCallback\(\)
		      .*/main\.go:27 \+0x[0-9,a-f]+
		  main._cgoexpwrap_[0-9a-z]+_goCallback\(\)
		      .*/_cgo_gotypes\.go:[0-9]+ \+0x[0-9,a-f]+
		
		Goroutine [0-9] \(running\) created at:
		  runtime\.newextram\(\)
		      .*/runtime/proc.go:[0-9]+ \+0x[0-9,a-f]+
		==================
		got:

NoRaceAtomicAddInt64                     .
RaceAtomicAddInt64                       .
NoRaceAtomicAddInt32                     .
[...]
NoRaceWaitGroupReuse2                    .
RaceWaitGroupReuse                       .
NoRaceWaitGroupConcurrentAdd             .

Passed 347 of 347 tests (100.00%, 0+, 0-)
0 expected failures (0 has not fail)
FAIL
FAIL	runtime/race	62.794s

/cc @dvyukov

[dvyukov]

dup of #14119