[llvm] [clang] [RISCV] Add experimental support of Zaamo and Zalrsc (PR #77424)

[Ved Shanbhogue via cfe-commits]

ved-rivos wrote:

> > > I guess Zaamo + Zacas is technically a way one could implement atomics without LR/SC?
> > 
> > 
> > The Zacas extension depends upon the A extension.
> 
> I filed an issue asking about that [riscv/riscv-zaamo-zalrsc#5](https://github.com/riscv/riscv-zaamo-zalrsc/issues/5)

The spec now clarifies that with Zaamo, Zacas depends on Zaamo.

https://github.com/llvm/llvm-project/pull/77424