[libc++][PSTL] Move all the OpenMP conditionals into a single block

All the used OpenMP pragmas are available with OpenMP 4.0, so they can all share a single conditional block.

Reviewed By: ldionne, #libc

Spies: yaxunl, sstefan1, guansong, jplehr, libcxx-commits, sunshaoce

Differential Revision: https://reviews.llvm.org/D149603