[source-build] We shouldn't hardcode certain architectures to be cross-compile only

https://github.com/dotnet/installer/blob/319b645486160238b2fab34753aa267a28ad07dd/src/SourceBuild/tarball/content/tools-local/init-build.proj#L119-L122

Code like this automatically assumes that if you are building on arm32, you must be cross compiling. We should handle that better, by looking at our build/current vs target architecture and then figuring out whether we need to enable cross compilation.