runtime/race: race detector can call exit more than once on finish

[jmchacon]

Seen on linux amd64 1.6 (but verified present at tip as well)

When using the race detector and C++ code is linked into the process as well it's possible for 2 threads to call racefini() today which eventually calls __tsan_fini which itself calls the regular C library exit().

Calling exit > 1 times is undefined behavior and means in a C++ program that the static destructors get invoked twice. On a glibc implementation of the exit handlers it also means a double free() occurs when the entry from the exit handlers list is freed upon completion.

[jmchacon]

I have a CL incoming to address this

[ianlancetaylor]

CC @dvyukov

[gopherbot]

CL https://golang.org/cl/22882 mentions this issue.