Android: Builds for arm64 default to incompatible Android 16 platform

When building for arm64-v8a, Android 21 is the minimum requirement in the NDK, but Qt will default to Android 16 (which is the minimum for 32-bit builds in Qt).

This causes very confusing build errors and is bound to cause a lot of problems, though the workaround is fairly simple: Just add -android-ndk-platform android-21 to the configure options.