Fix unmarshalling of gssize

Do not assume that the v_int union member always corresponds to a gssize. This
is not true on big-endian 64 bit machines like ppc64, so add a new
gi_argument_to_gssize() and use it properly.

https://bugzilla.gnome.org/show_bug.cgi?id=680693
https://bugzilla.redhat.com/show_bug.cgi?id=842880