[PATCH] [libc++] Allow libc++ to be built on systems without POSIX threads

[Jon Roelofs]

XFAILS for this are here: http://reviews.llvm.org/D5013

I didn't add any of the lists as a subscriber as this diff is rather large and mechanical.

In summary it adds "// XFAIL: libcxx-single-threaded" to all of the tests which this "breaks".

http://reviews.llvm.org/D3969