cmd/link/internal/ld: removed some uses of stringsCompare

cmd/link/internal/ld: removed some uses of stringsCompare

Only one use of stringsCompare is left. Cannot simply be replaced by
strings.Compare for bootstrapping reasons I guess.
Moving the function away from util.go to the actual destination data.go
also would not help much. So I left this one unchanged for readability and convenience.

Change-Id: I60d22fec0be8f8c47c80586436f9a550af59194e
Reviewed-on: https://go-review.googlesource.com/14953
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>