cmd/link: pass -Wl,-z,relro to host linker in -buildmode=PIE

Fixes #15485

Change-Id: I8e9314be91db89873130b232b589a284822e6643
Reviewed-on: https://go-review.googlesource.com/22687
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1091,6 +1091,9 @@ func hostlink() {
 			argv = append(argv, "-Wl,-pagezero_size,4000000")
 		}
 	case BuildmodePIE:
+		if UseRelro() {
+			argv = append(argv, "-Wl,-z,relro")
+		}
 		argv = append(argv, "-pie")
 	case BuildmodeCShared:
 		if HEADTYPE == obj.Hdarwin {