llext: add a relocatable extension test to Xtensa

LLEXT on Xtensa now supports both shared and relocatable (partially
linked) extensions. This commit adds a copy of the LLEXT test for the
relocatable case.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>