cmd/link: don't generate runtime.text twice for buildmode=plugin on darwin

https://golang.org/cl/29394 changed to include runtime.text and
runtime.etext in ctxt.Textp as a work around.
But it seems that the CL forgot to change genasmsym.
As a result, we are generating runtime.text and runtime.etext twice.

Change-Id: If7f8faf496c1c489ffa4804da712f91a3d3f4be4
Reviewed-on: https://go-review.googlesource.com/62810Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1939,8 +1939,13 @@ func genasmsym(ctxt *Link, put func(*Link, *Symbol, string, SymbolType, int64, *
 	// skip STEXT symbols. Normal STEXT symbols are emitted by walking textp.
 	s := ctxt.Syms.Lookup("runtime.text", 0)
 	if s.Type == STEXT {
+		// We've already included this symbol in ctxt.Textp
+		// if ctxt.DynlinkingGo() && Headtype == objabi.Hdarwin.
+		// See data.go:/textaddress
+		if !(ctxt.DynlinkingGo() && Headtype == objabi.Hdarwin) {
 			put(ctxt, s, s.Name, TextSym, s.Value, nil)
 		}
+	}
 
 	n := 0
 
@@ -1965,8 +1970,13 @@ func genasmsym(ctxt *Link, put func(*Link, *Symbol, string, SymbolType, int64, *
 
 	s = ctxt.Syms.Lookup("runtime.etext", 0)
 	if s.Type == STEXT {
+		// We've already included this symbol in ctxt.Textp
+		// if ctxt.DynlinkingGo() && Headtype == objabi.Hdarwin.
+		// See data.go:/textaddress
+		if !(ctxt.DynlinkingGo() && Headtype == objabi.Hdarwin) {
 			put(ctxt, s, s.Name, TextSym, s.Value, nil)
 		}
+	}
 
 	for _, s := range ctxt.Syms.Allsym {
 		if s.Attr.NotInSymbolTable() {