Ahaggar: add the walk-gimple header definition of gimple walk functions

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: define the walk GCC gimple functions

These functions will be used to walk some of the GCC gimple statements.
Not all gimple statements are supported but currently we implement only
the ones we need.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>