cmd/link: stop passing unused read_only_relocs

The external darwin linker has been printing:

	ld: warning: -read_only_relocs cannot be used with x86_64

for a long time. Now that it is printed by CL 33301, we may as
well get rid of it.

Fixes #20246

Change-Id: I1147cf1ff197fdfda228a1349f13627bcf9fc72f
Reviewed-on: https://go-review.googlesource.com/42730
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1075,7 +1075,10 @@ func (l *Link) hostlink() {
 		argv = append(argv, "-pie")
 	case BuildmodeCShared:
 		if Headtype == objabi.Hdarwin {
-			argv = append(argv, "-dynamiclib", "-Wl,-read_only_relocs,suppress")
+			argv = append(argv, "-dynamiclib")
+			if SysArch.Family != sys.AMD64 {
+				argv = append(argv, "-Wl,-read_only_relocs,suppress")
+			}
 		} else {
 			// ELF.
 			argv = append(argv, "-Wl,-Bsymbolic")