"qmake -spec <mkspec>" fails for cross builds

Consider a cross-built Qt, like Qt for Android or Qt for iOS.
Trying to call qmake with the -spec argument will fail:

$ ~/Qt/6.0.3/android_arm64_v8a/bin/qmake -spec android-clang 
Could not find qmake spec '-qtconf'.
Error processing project file: /home/sven/Qt/6.0.3/android_arm64_v8a/bin/target_qt.conf
Cannot find file: android-clang.

This happens, because:

• the qmake in Qt for Android is a wrapper script that calls
  qmake -qtconf qt_target.conf -spec android-clang
• the first stage of command line argument handling in qmake garbles the call to
  qmake -spec -qtconf qt_target.conf android-clang