llext: make local relocations optional

Some applications can decide to link their loadable objects for
exactly the same addresses, where they will be loaded. In those cases
local relocations aren't needed any more and can in fact break the
object if applied while the object is in a temporary storage. Add a
parameter to skip such local relocations.

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