eco32/gas: make a hack slightly less hacky

The off-by-one error that this hack fixes is actually an
off-by-one *word* error, so apply the hack as such.
eco32: implement elf_backend_relocate_section

This fixes several issues.

First, the addend for relocations relative to the start of a
section wouldn't be updated when linking objects together
in several steps.
E.g.
'ld -r -o obj3-1.o obj1-1.o obj2-1.o'
'ld -r -o obj3-2.o obj1-1.o obj2-1.o'
'ld -r -o obj3.o obj3-1.o obj3-2.o'

Second, the generic implementation crashed on lto sections.