BFD: Rename `*_set_reloc' to `*_finalize_section_relocs'

The `*_set_reloc' interface is to be called at the conclusion of section
relocation processing, however its name reflects a particular action to
take rather than the context of invocation.  Implementation is already
backend-specific.

Rename the interface such as not to make its name artificially limit the
intended purpose.  Update the callers and documentation accordingly.  No
functional change.