Raise exception when RV32E instructions use x16-x31

Modify rv_decode() to enforce RV32E register constraints by validating
rd, rs1, rs2, and rs3, ensuring they remain within x0-x15. If an
instruction attempts to use x16-x31, trigger an illegal instruction
exception, aligning with RISC-V privileged architecture behavior.

According to The RISC-V Instruction Set Manual Volume I: Unprivileged
Architecture, Version 20240411:

"RV32E and RV64E ...  only registers x0-x15 are provided. All encodings
specifying the other registers x16-x31 are reserved."

Author: eleanorLYJ

src/decode.c

@@ -2026,7 +2026,16 @@ bool rv_decode(rv_insn_t *ir, uint32_t insn)
         /* decode instruction (compressed instructions) */
         const decode_t op = rvc_jump_table[c_index];
         assert(op);
+#if !RV32_HAS(RV32E)
         return op(ir, insn);
+#else
+        if (!op(ir, insn))
+            return false;
+
+        if (ir->rd > 15 || ir->rs1 > 15 || ir->rs2 > 15 || ir->rs3 > 15)
+            return false;
+        return true;
+#endif
     }
 #endif
 
@@ -2036,7 +2045,14 @@ bool rv_decode(rv_insn_t *ir, uint32_t insn)
     /* decode instruction */
     const decode_t op = rv_jump_table[index];
     assert(op);
-    return op(ir, insn);
+    if (!op(ir, insn))
+        return false;
+
+#if RV32_HAS(RV32E)
+    if (ir->rd > 15 || ir->rs1 > 15 || ir->rs2 > 15 || ir->rs3 > 15)
+        return false;
+#endif
+    return true;
 
 #undef OP_UNIMP
 #undef OP