[libc++] Rename PS() macro to avoid clashing with Xtensa register name

This patch addresses a clash with the PS register from Xtensa
defined in the <specreg.h> header file, which is commonly included in
OS implementation.

Issue identified while building libc++ port for Apache NuttX, targeting
Xtensa-based chips (e.g. Espressif's ESP32).

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>

Differential Revision: https://reviews.llvm.org/D122479