newlib: enable timeradd, timersub, and similar macros
newlib: enable timeradd, timersub, and similar macros

This uses a hack (temporarily defining __rtems__ from a wrapper header
file) to include timeradd, timersub, and similar macros, without
modifying newlib headers.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf