llext: fix section addresses at link time by using sh_info

Currently, the code uses the section name to identify the target section
of a relocation. This is not reliable, as the section name is not
guaranteed to be in a specific format. Instead, use the sh_info field of
the relocation section header to identify the target section.

This is a tricky change, as it requires a workaround for the Xtensa
port, whose code path diverges here into the `link_plt` function and
ultimately different arch-specific code. Avoiding this divergence
will require additional refactorings.

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