crunchide(1): support non-custom elf object layout

The crunchide utility presumes the last 3 chunks of an ELF object
layout are section headers, symbol table, and then string table.
However, this is not specified in the ELF standards, and linkers
may generate different layouts when doing partial linking (-r).

This change is required to build FreeBSD with mclinker or the
gold linker.

PR: bin/174011
Submitted by: Pete Chou
Reviewed by: Cristoph Mallon
MFC after: 2 weeks