cmd/link: correct Mach-O file flag

Only set MH_NOUNDEFS if there are no undefined symbols.
Doesn't seem to matter, but may as well do it right.

Change-Id: I6c472e000578346c28cf0e10f24f870e3a0de628
Reviewed-on: https://go-review.googlesource.com/55310Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -263,7 +263,11 @@ func machowrite() int {
 	}
 	Thearch.Lput(uint32(len(load)) + uint32(nseg) + uint32(ndebug))
 	Thearch.Lput(uint32(loadsize))
+	if nkind[SymKindUndef] == 0 {
 		Thearch.Lput(MH_NOUNDEFS) /* flags - no undefines */
+	} else {
+		Thearch.Lput(0) /* flags */
+	}
 	if macho64 {
 		Thearch.Lput(0) /* reserved */
 	}