ILLink is no longer crossgen'd

With the change to download a linker package that matches the target framework (https://github.com/dotnet/sdk/pull/29441), ILLink is no longer bundled with the SDK and therefore is no longer going to be crossgen'd (see https://github.com/dotnet/installer/pull/15151#issuecomment-1363190864).

Opening this as a tracking issue in case we want to do something about that. Let's keep an eye on the build performance. If we need to, we could probably ship RID-specific ILLink packages that are crossgen'd as part of the runtime build.