csr: validate CSRRS/CSRRC writes by register index

[carlosqwqqwq]

csr: validate CSRRS/CSRRC writes by register index

Fixes #293

Commit message

csr: validate CSRRS/CSRRC writes by register index

Description

The CSR helper decides whether CSRRS/CSRRC writes from the runtime value loaded from rs1. A nonzero rs1 register containing zero is therefore mistakenly treated as a read, allowing writes to read-only time. Carry the architectural write-intent bit from the decoder into the helper: only rs1=x0/zimm=0 forms are read-only accesses.

Validation

• Native RISC-V hardware and QEMU trap csrrs/csrrc with rs1 = t0 (value zero) against read-only time and accept only the canonical rs1 = x0 read form; the patched build matches both.