There is no 3rd-party toolchain that you can use for a complete iOS (or even MacOS) build. Even if you cross-compile the majority of your code, you will need to transfer the binaries to a Mac for code signing. Everywhere I've worked that has produced Mac builds has had at least a small cluster of Mac machines to perform code signing. (But usually at this point, you just give up and perform the entire build on the Mac cluster.)