riscv: write zero for default Zimop MOPs

[carlosqwqqwq]

riscv: write zero for default Zimop MOPs

Fixes #305

Commit message

riscv: write zero for default Zimop MOPs

Description

The accepted mop.r/mop.rr decode path returns without performing the architectural default writeback. The Zimop default operation writes zero to x[rd]; add that writeback while leaving extension-admission policy unchanged. The change is limited to src/cpu/riscv_priv.c and applies independently to the staging branch.

Validation

• Native RISC-V hardware and QEMU are the references; disabled-extension code points trap, while the default Zimop operation writes rd = 0 on the mop.r.0, mop.r.31, mop.rr.0 and mop.rr.7 witnesses.