llext: merge llext_mem and llext_section enums

The only difference in the two enums are some entries related to
relocation sections. However, these entries are not used in the
code, so they can be safely removed, along with the mapping function.

Use LLEXT_MEM_* to avoid confusion with low-level "section" names.

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