newlib: Avoid introducing ESP-IDF macros from newlib platform headers
newlib: Avoid introducing ESP-IDF macros from newlib platform headers

Also, rewrite the assert.h header to be clearer

Closes https://github.com/espressif/esp-idf/issues/6445

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