objcgen does not support netcore assemblies by default

Steps to Reproduce

I've created vanilla netcore app with "hello world" class, built it with Visual Studio and trying to use objcgen from terminal:

~/.nuget/packages/embeddinator-4000/0.4.0/tools/objcgen net-embedding/bin/Debug/netcoreapp3.1/net-embedding.dll --out=out --platform=macOS-modern --abi=x86_64 --nativeexception --compile --debug --target=framework
Symbol file /Users/denis.lebedev/.nuget/packages/embeddinator-4000/0.4.0/tools/IKVM.Reflection.pdb doesn't match image /Users/denis.lebedev/.nuget/packages/embeddinator-4000/0.4.0/tools/IKVM.Reflection.dll
Parsing assemblies...
	Parsed 'net-embedding/bin/Release/netcoreapp3.1/net-embedding.dll'
Processing assemblies...
error EM0013: Can't find the assembly 'System.Runtime, Version=4.2.2.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a', referenced by 'net-embedding, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
Debug Log:
Processing: 1 assemblies
Processing Assembly: net-embedding

Expected Behavior

Framework is built

Actual Behavior

No framework

Environment

macos 10.15.5 Visual Studio 8.6.5 objcgen 0.4.0