cmd/link/internal/ld: move ld.Cpos to coutbuf.Offset
This change moves the ld.Cpos function to a method on coutbuf. This is
part of a larger change that makes ld.outbuf look more like a bio.Buf in
an effort to eventually replace the former with the latter.
Change-Id: I506f7131935a2aa903fa302a0fab0c5be50220fd
Reviewed-on: https://go-review.googlesource.com/27578
Run-TryBot: Dave Cheney <dave@cheney.net> Reviewed-by: David Crawshaw <crawshaw@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>