In principle you can, the Mach-O format is openly documented and implemented in the major compilers. The issue is that you need a sysroot (aka SDK) of the frameworks and headers for your target OS, which in Apple's case are proprietary and cannot be redistributed legally (you could probably rip them out of a macOS installation yourself though). For iOS apps you'd also need to sign the binaries and install the app to the device which is non-trivial to impossible to do on other platforms.