[Wasm-GC] Fix missing write barrier in BBQJIT struct.set

[Asumu Takikawa]

The following test crashes in the BBQJIT tier:

```
//@ runWebAssemblySuite("--useWebAssemblyTypedFunctionReferences=true", "--useWebAssemblyGC=true")

import * as assert from "../assert.js";
import { compile, instantiate } from "./wast-wrapper.js";

const m1 = instantiate(`
  (module
    (type (struct (field i32)))
    (type (struct (field (mut (ref null 0)))))
    (func (export "f") (result (ref any))
      (struct.new 1 (ref.null 0)))
  )
`);

const struct = m1.exports.f()
gc();

const m2 = instantiate(`
  (module
    (type (struct (field i32)))
    (type (struct (field (mut (ref null 0)))))
    (func (export "g") (param (ref 1))
      (struct.set 1 0 (local.get 0) (struct.new 0 (i32.const 42))))
    (func (export "h") (param (ref 1)) (result i32)
      (struct.get 0 0 (struct.get 1 0 (local.get 0))))
  )
`);

m2.exports.g(struct);
edenGC();
assert.equals(m2.exports.h(struct), 42);
```
This is due to a missing write barrier, ensuring that the old object from module m1 has a reference to a new object in module m2 that will be traced.

[Asumu Takikawa]

Pull request: https://github.com/WebKit/WebKit/pull/21484

[EWS]

Committed 271740@main (50def56d3557): <https://commits.webkit.org/271740@main>

Reviewed commits have been landed. Closing PR #21484 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/119382549>