iOS: Remove need for -force_load of platform plugin

Instead of force-loading the whole static library of the platform plugin
we tell the linker to look for the missing symbol qt_registerPlatformPlugin.
This symbol is provided by the same object file as the plugin's static
initializer, so the object file is included in the final binary and
the static initializer is run, resulting in the plugin registering with
Qt.

We could have marked the actual static initializer wrapper provided by
Q_IMPORT_PLUGIN(QIOSIntegrationPlugin) as undefined, but due to the C++
mangling this would look less intuitive on the linker command line than
the custom dummy function that we provide, which has C linkage.

Change-Id: I6805537e1f49260a41d48c555376964cb1fe75d8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
iOS: Remove need for separate qtiosmain library

We can combine the hybrid and non-hybrid use-cases into a single static
library if we are careful about which symbols are included in which
object files. By limiting the main() and qt_user_main() functions to
their own translation units, the linker will only pick them up if they
are missing at link time (the user's program do not provide them).

This technique is resilient to the -ObjC linker flag, which includes all
object files that implement an ObjectiveC class or category, but will
fail if the -all_load flag is passed to the linker, as we'll then have
duplicate symbols for either main() or qt_user_main(). The latter should
not happen unless the user provides the flag manually, and in the case
he or she does, there's ways to work around it by providing less global
flags such as -ObjC or -force_load.

Change-Id: Ie2f8e10a7265d007bf45cb1dd83f19cff0693551
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>