llext: define target binary type

Add a new Kconfig option to select the binary object type for the llext
subsystem. This will allow to fully decouple the architecture type from
the kind of binary object that is expected by the loader.

The defaults have been chosen to match the current behavior of the ARM
and Xtensa architectures, but developers can now more easily experiment
with other object types.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>