Can we deprecate LINKABLE and RELOCATABLE settings

These settings I believe were created as part of enabled dynamic linking, but its not clear that they are useful on their own.

Being able to remove these settings or at least make them internal would allow for bunch of simplification. Especially since we are trying to move towards a MAIN_MODULE that is *not* RELOCATABLE, keeping the public RELOCATABLE setting around would really complicate things in that world.

See https://github.com/emscripten-core/emscripten/issues/12682