[libc++][Modules] Add missing submodules

Several headers that are included by the modular headers are missing from the module map, add those in.
The either/or implementation headers `<__algorithm/pstl_backends/cpu_backends/serial.h>`/`<__algorithm/pstl_backends/cpu_backends/thread.h>` need to be textual, as does `<__undef_macros>`.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D153213