[RISCV][MC] Add experimental support of Zaamo and Zalrsc

`A` extension has been split into two parts: Zaamo (Atomic Memory
Operations) and Zalrsc (Load-Reserved/Store-Conditional). See also
https://github.com/riscv/riscv-zaamo-zalrsc.

This patch adds the MC support.

Reviewers: dtcxzyw, topperc, kito-cheng

Reviewed By: topperc

Pull Request: https://github.com/llvm/llvm-project/pull/78970