gtk-immodules-cache.bbclass: allow for offline cache generation
In order to support a RO rootfs, the cache generation during postinstall
has to be done on host. However, gtk-query-immodules application will
only be able to parse shared objects from the same ELF class.
In order not to have a native package for all the recipes providing
an input method, so we can generate the cache file natively, run
gtk-query-immodules through qemu emulator.
