llext: use CMake shared library support on Xtensa

This change reworks the Xtensa support in llext to use CMake's native
shared library support, instead of manually running "gcc -shared".

This change minimizes the differences in llext handling by defining
appropriate CMake targets for the different architectures.

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