RFC: target/riscv: Add support for Zacas extension

[Rob Bradford]

The Zacas[1] extension is a proposed unprivileged ISA extension for
adding support for atomic compare-and-swap. Since this extension is not
yet frozen (although no significant changes are expected) these patches
are RFC/informational.

Cheers,

Rob

[1] - https://github.com/riscv/riscv-zacas/blob/main/zacas.adoc