Temporarily exclude NativeAOT from source-build until we figure out how to handle it

[crummel]

As-is, we don't believe NativeAOT will work in source-build anyway. Removing it from the build while we decide what to do removes a prebuilt.

See #69611 for further discussion.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[MichalStrehovsky]

Is it difficult to include llvm-project in source build? It sounds like there are two issues with NativeAOT and source build:

• Is runtime.linux-x64.Microsoft.NETCore.Runtime.ObjWriter necessary for source-build? #69611 - would be fixed by including llvm-project in source build
• NativeAOT doesn't work with Linux non-portable build #66859 - Jan sketched out a fix for that in Is runtime.linux-x64.Microsoft.NETCore.Runtime.ObjWriter necessary for source-build? #69611 (comment) and that one doesn't seem difficult.

Cc @agocke

[MichalStrehovsky]

When it comes to the build time of LLVM, objwriter only builds a small subset of LLVM and we can also do sparse checkout like this:

https://github.com/dotnet/runtimelab/blob/b2dc37ba181a7fa4427e717eab819ba3543d0ae4/src/coreclr/tools/aot/ObjWriter/build.cmd#L24-L29

It builds in about 5 minutes on my 7 year old laptop on Windows.

[jkotas]

This will also exclude crossgen2 that you want to keep for source build. This would need to be done differently.

[crummel]

I think including the minimal subset of llvm-project for the ObjWriter package sounds reasonable and I'll give this a try. Closing this issue for now.