[release-branch.go1.8] cmd/link: do not pass -s through to host linker on macOS

[release-branch.go1.8] cmd/link: do not pass -s through to host linker on macOS

This keeps the host linker from printing
ld: warning: option -s is obsolete and being ignored

Fixes #19775.

Change-Id: I18dd4e4b3f59cbf35dad770fd65e6baea5a7347f
Reviewed-on: https://go-review.googlesource.com/38851
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/39606
TryBot-Result: Gobot Gobot <gobot@golang.org>