liblink, cmd/gc, cmd/{5,6,8}{a,c}: rename linkwriteobj to writeobj

liblink, cmd/gc, cmd/{5,6,8}{a,c}: rename linkwriteobj to writeobj

The name linkwriteobj is misleading because it implies that
the function has something to do with the linker, which it
does not.  The name is historical: the function performs an
operation that was previously performed by the linker, but no
longer is.

LGTM=rsc
R=rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/88210045