system: fix brownout ISR triggering assert on single-core configs.
system: fix brownout ISR triggering assert on single-core configs.

ISR handler was incorrectly calling stall other cpu even on single core systems

Closes https://github.com/espressif/esp-idf/issues/9456

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