title: xcodebuild error while generating xcframework library
body: <p>I'm trying to create xcframework of j2objc built by make dist. </p>
  
  <p>Presumerbly those .a files are compiled as fat library, but built with xCode10 due to compile errors when compile with xCode11. </p>
  
  <p>My build environments are xCode11 beta 2 and MacOS 10.5 beta.</p>
  
  <pre><code>$ xcodebuild -create-xcframework -library ./lib/libjre_emul.a -library ./lib/macosx/libjre_emul.a -headers ./include/ -output lib/universal/libjre_emul.xcframework
  
  error: binaries with multiple platforms are not supported './j2objc/dist/lib/libjre_emul.a'
  </code></pre>