racefini calls __tsan_fini which is C code and at the end of it
invoked the standard C library exit(3) call. This has undefined
behavior if invoked more than once. Specifically in C++ programs
it caused static destructors to run twice. At least on glibc
impls it also means the at_exit handlers list (where those are
stored) also free's a list entry when it completes these. So invoking
twice results in a double free at exit which trips debug memory
allocation tracking.

Fix all of this by using an atomic as a boolean barrier around
calls to racefini being invoked > 1 time.

Fixes #15578

Change-Id: I49222aa9b8ded77160931f46434c61a8379570fc
Reviewed-on: https://go-review.googlesource.com/22882Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>