cmd/link: support wasmexport on js/wasm

Add export functions to the wasm module on GOOS=js. (Other parts
work the same way as wasip1.)

Add a test.

Fixes #65199.

Change-Id: Ia22580859fe40631d487f70ee293c12867e0c988
Reviewed-on: https://go-review.googlesource.com/c/go/+/606855


Reviewed-by: Zxilly Chou <zxilly@outlook.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>