fix(asm): reject the operand shapes the toolchain rejects on riscv64

Assisted-by: GLM 5.3 Flash

asm/riscv_toolchain_test.go

@@ -209,12 +209,12 @@ func TestRISCVToolchainRejections(t *testing.T) {
 		{
 			name: "fence predecessor",
 			src:  "\tFENCE X, W\n",
-			want: "FENCE: invalid predecessor operand",
+			want: "invalid FENCE predecessor operand",
 		},
 		{
 			name: "fence successor",
 			src:  "\tFENCE W, Q\n",
-			want: "FENCE: invalid successor operand",
+			want: "invalid FENCE successor operand",
 		},
 		{
 			name: "immediate CSR pseudo with register",