linux-user/FLAT: allow targets to override FLAT processing

This brings flatload.c more in line with the current Linux FLAT loader
which allows targets to handle various FLAT aspects in their own way.
For the common behavior, the new functions get stubbed out.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
