Check/fix assemblies being invalid as far as netcore/netfw runtimes are concerned

[knah]

Last time I checked, netcore/netfw runtimes refused to load generated assemblies. One specific issues was enum types having the Il2CppType static field. After the field is removed in one of the future updates, we need to investigate if there's anything else invalid in generated assemblies, and fix those issues to make the assemblies compatible with netcore.

Mono somehow cares way less about what's in assemblies it loads, so current assemblies work just fine on mono.