Merging r285782:

------------------------------------------------------------------------
r285782 | brad | 2016-11-01 18:39:01 -0700 (Tue, 01 Nov 2016) | 9 lines

Disable the use of std::call_once on OpenBSD with libstdc++.

It was noticed this caused performance regressions and deadlocks. PR30768.

Reorder the code to make it clearer what is tested.

PPC now disables the use of std::call_once only with libstdc++ with
the reordering of the code, as was the original intent.

------------------------------------------------------------------------

llvm-svn: 288075