[dev.link] cmd/link/internal/loader: copy Dynid when migrating sym attrs

Add support for migrating over the symbol Dynid property
when converting loader.Sym symbols to sym.Symbol.

Change-Id: Icc3b91b4adcae6f2ede7d915bb674cc206025217
Reviewed-on: https://go-review.googlesource.com/c/go/+/226360
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/loader/loader.go

@@ -2364,6 +2364,11 @@ func (l *Loader) migrateAttributes(src Sym, dst *sym.Symbol) {
 	if got, ok := l.got[src]; ok {
 		dst.SetGot(got)
 	}
+
+	// Copy dynid
+	if dynid, ok := l.dynid[src]; ok {
+		dst.Dynid = dynid
+	}
 }
 
 // CreateExtSym creates a new external symbol with the specified name