riscv: trap reserved C.LUI/C.ADDI16SP encodings instead of mis-decoding them

[carlosqwqqwq]

Fix: trap reserved C.LUI/C.ADDI16SP and C.ADDIW rd=x0 encodings

Problem

riscv_emulate_c_c1() in src/cpu/riscv_compressed.h falls through from the
reserved c.lui/c.addi16sp/c.mop code points into the c.misc-alu
decoder, executing reserved encodings as c.srli/c.srai/c.andi/c.sub-
family instructions. The RV64 c.addiw rd == x0 code points are also
reserved per the spec ("valid only when rd != x0") and fell through the same
way.

Change

Both affected paths now return explicitly instead of falling through:

• case 0x01 (RV64): c.addiw with rd == x0 (reserved) -> riscv_illegal_insn()
• case 0x03: c.lui/c.addi16sp with nzimm == 0 that is not a valid c.mop.N
  -> riscv_illegal_insn()

Verification

• 24 reserved c.lui-slot encodings: trap on native RISC-V hardware (SIGILL), QEMU
  (SIGILL), and now RVVM (both JIT and interpreter).
• RV64 c.addiw x0, imm reserved encodings: now raise an illegal-instruction
  trap, matching the spec and QEMU (c64_illegal).
• Regression: c.lui x0, 1 hint, c.mop.N (Zcmop), c.addiw with rd != 0,
  and all other C1 encodings still execute correctly.

Fixes #284.

[LekKit]

How come is c.addiw with rd = 0 a valid instruction? The documentation says:

C.ADDIW is only valid when rd≠x0; the code points with rd=x0 are reserved.

https://docs.riscv.org/reference/isa/v20260120/unpriv/c-st-ext.html#27-1-5-2-integer-register-immediate-operations

[LekKit]

Other than that, the fix is sound. Previously both cases fell through into riscv_illegal_insn(vm, insn);, but the interpreter code was moved around and now incorrectly decodes them

[carlosqwqqwq]

You are right - thank you for the catch. I checked the spec: C.ADDIW is valid only when rd != x0; the rd == x0 code points are reserved (not HINTs), and QEMU decodes them as c64_illegal. I have updated the patch so c.addiw rd == x0 now raises an illegal-instruction trap instead of being treated as a no-op, and force-pushed. The c.lui/c.addi16sp nzimm == 0 slot (the main subject of the PR) is unchanged.