cmd/compile: do not emit args_stackmap for func _

Fixes #11699.

Change-Id: I01bf506d76260bcdf828bbde52791e328aa441a5
Reviewed-on: https://go-review.googlesource.com/16921Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/pgen.go

@@ -129,6 +129,9 @@ func gcsymdup(s *Sym) {
 }
 
 func emitptrargsmap() {
+	if Curfn.Func.Nname.Sym.Name == "_" {
+		return
+	}
 	sym := Lookup(fmt.Sprintf("%s.args_stackmap", Curfn.Func.Nname.Sym.Name))
 
 	nptr := int(Curfn.Type.Argwid / int64(Widthptr))

test/fixedbugs/issue11699.go

+// compile
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// issue 11699; used to fail with duplicate _.args_stackmap symbols.
+
+package p
+
+func _()
+func _()