newlib: initialize syscall_table_ptr_app in single core mode
newlib: initialize syscall_table_ptr_app in single core mode

ESP32 ECO3 ROM uses syscall_table_ptr_app on both CPUs, so the pointer
needs to be set if running in single core mode.

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