Add support for (register) command in spec tests and enable importing mutable globals.

Enable the part of the simd spec tests that now pass.

lib/runtime-core/src/codegen.rs

@@ -146,7 +146,7 @@ pub fn validating_parser_config(features: &Features) -> wasmparser::ValidatingPa
             enable_bulk_memory: false,
             enable_multi_value: false,
         },
-        mutable_global_imports: false,
+        mutable_global_imports: true,
     }
 }