package/pkg-generic.mk: move pkg_size_{before, after} and check_bin_arch functions

These functions are no longer using the GLOBAL_INSTRUMENTATION_HOOKS
mechanism, so it doesn't make much sense for them to be in the section
of pkg-generic.mk related to those hooks.

Move them to the "Helper functions" section.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>