How can I toggle a pin that doesn't have a GPIO alternate function?

For example, pin 87 (nRESET_OUT). Are the pins mapped to a memory location I can write to/read from?

Pins without GPIO alternate function can usually not be used. nRESET_OUT is a signal which is generated by hardware and software cannot influence this. Other pins w/o GPIO feature will directly be controlled by their assigned function, e.g USB, Ethernet, etc. It’s not possible to control such pins other than through their intended function.
What would you like to achieve? Maybe there are other options.