llext: support multiple source files

This change allows the `add_llext_target` function to accept multiple
source files when building an ELF shared library. The ELF object
target type is still limited to a single source file, since there is no
linking step in that case.

Also fixes a minor typo in another llext function documentation.

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