[ELF] Move SHT_REL/SHT_RELA handling from createInputSection to initializeSections

This simplifies the code a bit. While here,

* change the `multiple relocation sections` diagnostic from `fatal` to `error` and include the relocated section name.
* drop less useful name from `getRelocTarget`. Without -r/--emit-relocs we don't need to get SHT_REL/SHT_RELA names.