[EH] Use random value for exnref encoding when legacy GC is used

[aheejin]

Currently the legacy GC encoding's nullexternref encoding overlaps with exnref's. We assume the legacy GC encoding won't be used with the exnref for the moment and assign a random value to it to prevent the clash.

[aheejin]

alpine CI fails with:

Run ./alpine.sh pip3 install -r requirements-dev.txt
error: externally-managed-environment

× This environment is externally managed
╰─> 
    The system-wide python installation should be maintained using the system
    package manager (apk) only.
    
    If the package in question is not packaged already (and hence installable via
    "apk add py3-somepackage"), please consider installing it inside a virtual
    environment, e.g.:
    
    python3 -m venv /path/to/venv
    . /path/to/venv/bin/activate
    pip install mypackage
    
    To exit the virtual environment, run:
    
    deactivate
    
    The virtual environment is not deleted, and can be re-entered by re-sourcing
    the activate file.
    
    To automatically manage virtual environments, consider using pipx (from the
    pipx package).

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP [6](https://github.com/WebAssembly/binaryen/actions/runs/7174694536/job/19536698686?pr=6166#step:6:7)6[8](https://github.com/WebAssembly/binaryen/actions/runs/7174694536/job/19536698686?pr=6166#step:6:9) for the detailed specification.
Error: Process completed with exit code 1.

https://github.com/WebAssembly/binaryen/actions/runs/7174694536/job/19536698686?pr=6166

Anyone knows what this is about?

[kripken]

I've never seen that before but it just happened on my PR too...