On Windows, CreateThread occasionally fails with ERROR_ACCESS_DENIED.
We're not sure why this is, but the Wine source code suggests that
this can happen when there's a concurrent CreateThread and ExitProcess
in the same process.

Fix this by setting a flag right before calling ExitProcess and
halting if CreateThread fails and this flag is set.

Updates #18253 (might fix it, but we're not sure this is the issue and
can't reproduce it on demand).

Change-Id: I1945b989e73a16cf28a35bf2613ffab07577ed4e
Reviewed-on: https://go-review.googlesource.com/34616
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>