cerbero: Simplify windows bootstrap a bit
cerbero: Simplify windows bootstrap a bit

fix_mingw() should only run when platform == windows, and we do not
need to delete `/mingw/lib` when running on 32-bit Windows because we
do not support building on 32-bit Windows at all now.

nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero