RISC-V: Add Zawrs ISA extension support

This patch adds support for the Zawrs ISA extension
("wrs.nto" and "wrs.sto" instructions).

The specification can be found here:
https://github.com/riscv/riscv-zawrs/blob/main/zawrs.adoc



Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>