gatomic: statically assert that our assumptions hold

This code assumes that int is exactly 4 bytes, and that pointers
are either 4 or 8 bytes, on platforms with __ATOMIC_SEQ_CST.
In practice this is going to be true.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=730932
glib-init: statically assert that we have 8-bit bytes

configure.ac assumes this.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=730932