cmd/link: allow duplicated weak symbols on Mach-O

This fixes cgo test issue29563 on Darwin.

Updates #29563.

Change-Id: If480078461247cd7c95931ae3ad4ca89736dd550
Reviewed-on: https://go-review.googlesource.com/c/go/+/170015
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/loadmacho/ldmacho.go

@@ -43,11 +43,6 @@ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
-const (
-	N_EXT  = 0x01
-	N_TYPE = 0x1e
-	N_STAB = 0xe0
-)
 
 // TODO(crawshaw): de-duplicate these symbols with cmd/internal/ld
 const (
@@ -161,6 +156,19 @@ type ldMachoDysymtab struct {
 	indir          []uint32
 }
 
+// ldMachoSym.type_
+const (
+	N_EXT  = 0x01
+	N_TYPE = 0x1e
+	N_STAB = 0xe0
+)
+
+// ldMachoSym.desc
+const (
+	N_WEAK_REF = 0x40
+	N_WEAK_DEF = 0x80
+)
+
 const (
 	LdMachoCpuVax         = 1
 	LdMachoCpu68000       = 6
@@ -616,6 +624,9 @@ func Load(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length i
 		if machsym.type_&N_EXT == 0 {
 			s.Attr |= sym.AttrDuplicateOK
 		}
+		if machsym.desc&(N_WEAK_REF|N_WEAK_DEF) != 0 {
+			s.Attr |= sym.AttrDuplicateOK
+		}
 		machsym.sym = s
 		if machsym.sectnum == 0 { // undefined
 			continue