test: skip a test on failing test on nacl/386

This test was designed for #15609 and didn't consider nacl. It's not
worth adding new +build-guarded assembly files in issue15609.dir for
nacl, especially as nacl is going away.

Fixes #32206

Change-Id: Ic5bd48b4f790a1f7019100b8a72d4688df75512f
Reviewed-on: https://go-review.googlesource.com/c/go/+/178698
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue15609.go

 // runindir
 
+// +build !nacl
+
 // Copyright 2019 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.