libc++: support NaCl when building thread.cpp

Summary: NaCl shouldn't include sysctl.h when trying to determine std::thread::hardware_concurrency, it should instead use sysconf(_SC_NPROCESSORS_ONLN) through unistd.h. No test needs to be changed, since hardware_concurrency.pass.cpp already tests that std::thread::hardware_concurrency > 0.

Test Plan: make check-libcxx

Reviewers: dschuff, danalbert

Subscribers: jfb, cfe-commits

Differential Revision: http://reviews.llvm.org/D6470