llext: export all Z_DEVICE_DEFINE devices

Some device types (SPI, network, etc) use customized versions of
DEVICE_DT_DEFINE; however, all of these ultimately use Z_DEVICE_DEFINE,
which is where the object that has to be exported is defined.

Move the Z_DEVICE_EXPORT macro to Z_DEVICE_DEFINE and make it depend on
'node_id' being valid, so that it properly covers all exportable devices
defined in the build.

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