llext: avoid direct llext_string() usage

Since 3466dab8 the generic llext_symbol_name() function abstracts
the use of llext_string() for (non-section) symbols. Define a similar
llext_section_name() function and replace current occurrences of
llext_string() with the proper abstraction.

By extending llext_symbol_name(), this commit also allows to print the
correct name of sections that are referred to by a symbol.

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