llext: fix llext_load() optimization

The optimization in llext_load() to avoid using the generic path for
sections that are cached in memory was broken for two reasons:
- it was comparing an ELF section index to LLEXT_MEM_BSS, which is a
  llext_mem enum, and
- it was using the wrong section address for the cached sections since
  the "merged sections" feature was introduced in 709b2e44.

This patch fixes both issues using the new llext_loaded_sect_ptr()
helper function.

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