[libc++][PSTL] Add a simple std::thread backend

[libc++][PSTL] Add a simple std::thread backend

This is just to test that the PSTL works with parallelization. This is not supposed to be a production-ready backend.

Reviewed By: ldionne, #libc

Spies: EricWF, arichardson, libcxx-commits

Differential Revision: https://reviews.llvm.org/D150284