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.
10 jobs for use-gobject-hole in 37 minutes and 9 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #414946
cross-android_api21_arm64

00:02:35

passed #414947
cross-android_api28_arm64

00:02:19

passed #414948
cross-mingw64

00:03:17

passed #414944
debian-stable-x86_64

00:03:41

passed #414943
fedora-x86_64

00:06:17

passed #414945
G_DISABLE_ASSERT

00:03:42

passed #414949
win32
msys2-mingw32

00:14:56

passed #414950
win32
vs2017-x64

00:09:47

 
  Coverage
passed #414951
coverage

00:01:13

77.9%
 
  Analysis
passed #414952
scan-build

00:04:54