llext: improve debug messages

The recent llext_map_sections() rework changed the way debug messages
are output so that the names of most skipped sections are not printed
at all. This makes debugging harder since the section names are useful
to identify the contents at a glance.

Also print a few additional fields from the section header, and use 0x
prefixes for hex numbers.

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