llext: (cosmetic) cleaner region overlap checks

Rewrite the overlap checks in llext_map_sections() to use a pair of
macros to calculate the top and bottom of a region. This makes the
code more readable and less error-prone.

No functional change is introduced in this commit apart from a log
message format change.

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