Disable relaxed-simd in differential fuzzing (bytecodealliance#10045)

We forgot to do this quite a long time ago but this was always the
intention. Turns out with Pulley now being online the fuzzer quickly
found a difference between Pulley and Cranelift where the native x64
instructions differ from the "deterministic" behavior that Pulley
implements. This difference is expected and allowed though, so don't
fuzz it.

crates/fuzzing/src/generators/config.rs

@@ -99,6 +99,10 @@ impl Config {
                 }
             }
         }
+
+        // These instructions are explicitly not expected to be exactly the same
+        // across engines. Don't fuzz them.
+        config.relaxed_simd_enabled = false;
     }
 
     /// Uses this configuration and the supplied source of data to generate