llext: Refactor symbol lookup logic into function

This commit refactors logic in llext_link.c that resolves
a symbol to be imported by an llext into a utility function.
Thereby, the logic can be re-used in other functions if
needed, e.g., when resolving symbols for indirect relocations.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>