cmd/link/internal/ld: don't use linkname before 1.12

Before the 1.12 release the use of linkname did not prevent a compiler
error for an empty function body (see #23311). Add some build tags so
that cmd/link will build with earlier releases.

It's true that we currently require Go 1.16 as the bootstrap compiler (#44505).
But for this simple case keep things working with older compilers for now.

Change-Id: I67fe021406096c64c01d6e2c9adbcc4388988a6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/355690
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/link/internal/ld/outbuf_darwin.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build darwin && go1.12
+// +build darwin,go1.12
+
 package ld
 
 import (

src/cmd/link/internal/ld/outbuf_nofallocate.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !darwin && !linux
-// +build !darwin,!linux
+//go:build (!darwin && !linux) || (darwin && !go1.12)
+// +build !darwin,!linux darwin,!go1.12
 
 package ld
 

src/cmd/link/internal/ld/outbuf_notdarwin.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !darwin
-// +build !darwin
+//go:build !darwin || (darwin && !go1.12)
+// +build !darwin darwin,!go1.12
 
 package ld