RVM: Zalrsc should be a valid optional extension

[ubc-guy]

RVM lists both A and Zaamo as valid optional extensions.

A = Zaamo + Zalrsc

it only makes sense that since A and Zaamo are optional, that Zalrsc can also be optional on its own.

Zaamo = easy to implement on simple cacheless microcontrollers (and doesn't need Zalrsc)

Zalrsc = might be easier to implement on high-performance multi-core cached microcontrollers (and doesn't need Zaamo)