[llvm-readelf/obj] - Index phdrs and relocations from 0 when reporting warnings.

As was mentioned in comments here:
https://reviews.llvm.org/D92636#inline-864967

we are not consistent and sometimes index things from 0, but sometimes
from 1 in warnings.

This patch fixes 2 places: messages reported for
program headers and messages reported for relocations.

Differential revision: https://reviews.llvm.org/D93805