[wasm-gc] Use correct Context when wrapping objects

For creating and unpacking the wrapper objects we currently use
for WasmGC <-> JavaScript interop, we must read the Context from
the "WasmApiFunctionRef" rather than from the "instance_node_".

As a bonus, this patch also slightly improves generated code
for wasm-to-js wrappers by moving conditionally needed code to
labels (previously it was always executed but the results only
conditionally used).

Fixed: v8:12640
Change-Id: I55485cdb6a402f32ddc42ec6316dcbe23f1fcccb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3489486
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79270}