cmd/link: ignore macho symbols in invalid sections

Restores linker behavior that existed before CL 68930.
Hopefully fixes the macOS 10.8 builder.

Change-Id: Ib8d6923b59543008cf71c4625addb4e2045cedf6
Reviewed-on: https://go-review.googlesource.com/69170
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>

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

@@ -627,7 +627,7 @@ func Load(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length i
 		sect := &c.seg.sect[machsym.sectnum-1]
 		outer := sect.sym
 		if outer == nil {
-			return errorf("reference to invalid section %s/%s", sect.segname, sect.name)
+			continue // ignore reference to invalid section
 		}
 
 		if s.Outer != nil {