llext: add support for global functions

Loadable modules can contain global (not "static") functions, even if
they aren't exported for use by other modules, e.g. when a module is
built from multiple .c files. Such functions are then also included
in link tables and have to be re-linked.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>