[wasm-gc][bug] Remove UNIMPLEMENTED to supress fuzzer errors

Bug: chromium:1082309, chromium:1082083
Change-Id: I616da287adce5db7952eadf11364a7593e405fdb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2199346Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67791}

src/wasm/function-body-decoder-impl.h

@@ -1597,7 +1597,8 @@ class WasmDecoder : public Decoder {
           case kExprRttGet:
           case kExprRttSub: {
             // TODO(7748): Impelement.
-            UNIMPLEMENTED();
+            decoder->error(pc, "rtt opcodes not impelemented yet");
+            return 2;
           }
 
           default: