gobject: Fix compilation on OS X/ppc64

Apple's GCC compilers cannot deal well with 64-bit pointers in
transparent unions on ppc64, so compilation of
_G_DEFINE_BOXED_TYPE_BEGIN was failing. Fortunately glib already
provides a fallback for compilers that can't deal with it; this adds
this specific case to the check.

https://bugzilla.gnome.org/show_bug.cgi?id=647145