sections.h: dereference_function_descriptor() returns void pointer

The function dereference_function_descriptor() takes on hppa64, ppc64
and ia64 a pointer to a function descriptor and returns a (void) pointer
to the dereferenced function.
To make cross-arch coding easier, on all other architectures the
dereference_function_descriptor() macro should return a void pointer
too.

Signed-off-by: Helge Deller <deller@gmx.de>