[HTML5] Fix GDNative compilation with emcc 2.0.19+
[HTML5] Fix GDNative compilation with emcc 2.0.19+

Add `WARN_ON_UNDEFINED_SYMBOLS=0` for the main module (which defines
`godot_js_main` as extern coming from the "side" module, i.e. the main
Godot binary).

(cherry picked from commit 14c057eab605e8018a1ac7f98ed0f16faeabf9a2)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot