This future-proofs the Chdr64 structure against later versions of ELF
defining this field and declutters the documentation without changing
the layout of the struct.

This structure does not exist in the current release, so this change
is safe.

Change-Id: I239aad7243ddaf063a1f8cd521d8a50b30413281
Reviewed-on: https://go-review.googlesource.com/18028Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>