[RISCV] Use Zacas for AtomicRMWInst::Nand i32 and XLen. (#80119)

We don't have an AMO instruction for Nand, so with the A extension we
use an LR/SC loop. If we have Zacas we can use a CAS loop instead.

According to the Zacas spec, a CAS loop scales to highly parallel
systems better than LR/SC.