cmd/link: don't link the same dylib multiple times

Also, unexport Machoadddynlib

n=`go test -c crypto/x509 && otool -l x509.test | grep libSystem | wc -l`

Before this CL, n = 3.
After this CL, n = 1.

on my environment.

Change-Id: Ic7b8157435cc85086404860dc6c84eb0aecc5d19
Reviewed-on: https://go-review.googlesource.com/44771Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -175,7 +175,7 @@ func loadcgo(ctxt *Link, file string, pkg string, p string) {
 				havedynamic = 1
 
 				if Headtype == objabi.Hdarwin {
-					Machoadddynlib(lib)
+					machoadddynlib(lib)
 				} else {
 					dynlib = append(dynlib, lib)
 				}

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

@@ -380,7 +380,12 @@ func (ctxt *Link) domacho() {
 	}
 }
 
-func Machoadddynlib(lib string) {
+func machoadddynlib(lib string) {
+	if seenlib[lib] || Linkmode == LinkExternal {
+		return
+	}
+	seenlib[lib] = true
+
 	// Will need to store the library name rounded up
 	// and 24 bytes of header metadata. If not enough
 	// space, grab another page of initial space at the