Disable the use of std::call_once on PowerPC due to an apparent bug in

libstdc++ (or in compilers, or somewhere, I can't track it down) that
causes unittests that use INITIALIZE_PASS to crash.

The analysis I've been able to do is that inside libstdc++'s
implementation of std::call_once, it uses pthread_once, and when that
returns an error code it throws std::system_error which then eventually
calls std::terminate.

Hopefully some of the folks who work on PPC can try to sort out what's
going on here. Until then, they'll have to use the fallback
implementation.

llvm-svn: 271821