Use the GObject hole on 64bit arches for some flags to improve performance

This uses a 32bit hole in the GObject structure on 64bit arches as a flag field which can be optionally used for some preformance hints.

Currently there is a flag that gets set any time you connect to a signal on a GObject which is used as early bailout for signal emissions, and using the flags field instead of a user-data for checking if a GObject is under construction.

Note, I don't as yet have any real performance measurements to ensure that this actually helps performance, thus marking this as WIP. Performance measurements in !1083 (comment 632508).