debug/elf: add relocation constants

Compiled from various tables found around the internet.

Some of these are used by cmd/link.

Change-Id: I258b25e694dbe91a61d675763f2c47ccc928fd70
Reviewed-on: https://go-review.googlesource.com/69012
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>