[release-branch.go1.21] cmd/link: don't generate DYSYMTAB when extern…

…al linking on Mach-O

When external linking, the external linker will generate it.

Updates #60694.
For #61229.

Change-Id: I086a7628dd9baa84b46315641746fc3640473f2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/502617
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit f55e7e1)
Reviewed-on: https://go-review.googlesource.com/c/go/+/513757

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

@@ -833,9 +833,9 @@ func asmbMacho(ctxt *Link) {
 		ml.data[2] = uint32(linkoff + s1 + s2 + s3 + s4 + s5) /* stroff */
 		ml.data[3] = uint32(s6)                               /* strsize */
 
-		machodysymtab(ctxt, linkoff+s1+s2)
-
 		if ctxt.LinkMode != LinkExternal {
+			machodysymtab(ctxt, linkoff+s1+s2)
+
 			ml := newMachoLoad(ctxt.Arch, LC_LOAD_DYLINKER, 6)
 			ml.data[0] = 12 /* offset to string */
 			stringtouint32(ml.data[1:], "/usr/lib/dyld")