Proposal: remove support for the node_syscall extension

The extension has been deprecated for about 3 years (since https://github.com/gopherjs/gopherjs/pull/1111), and has been disabled by default since then. So far I haven't heard of anyone asking to continue supporting it. This extension has known issues in the memory management department and doesn't support all types of arguments (e.g. https://github.com/gopherjs/gopherjs/issues/993). I am not even sure it builds under the modern node versions.

I would like to do the following:

1. Move the source code into a separate repository under the gopherjs organization and mark it archived. This will keep sources available in case anyone is using it for some other purpose, and it would clearly indicate its status an unmaintained.
2. Remove legacy_syscall build tag from the code base.
3. Remove the option of using different GOOS/GOARCH for user code which was added as a compatibility option in #1111 and fix them to GOOS=js GOARCH=ecmascript.

Inviting @flimzy and anyone else to comment.