Add a (temporary) assertion that sizeof(gsize) == sizeof(guintptr)

Glib does not yet have a well defined ABI/API for architectures
where gsize != guintptr (e.g. CHERI). To enable working on bringup
for new architectures this assertion can be disabled with
`-DG_ENABLE_EXPERIMENTAL_ABI_COMPILATION`.

See https://gitlab.gnome.org/GNOME/glib/-/issues/2842



[smcv: Apply review comments from pwithnall]

Signed-off-by: Simon McVittie <smcv@collabora.com>