x/tools/cmd/stress: don't build for Plan 9 (syscall.SIGABRT not defined)

See golang/go#11975.
For golang/go#11811.

Change-Id: I56ee20cd798bf963afdf3c81c4745f07850f6dcc
Reviewed-on: https://go-review.googlesource.com/13034
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cmd/stress/stress.go

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// TODO: syscall.SIGABRT is not defined for Plan 9 (issue #11975)
+
+// +build !plan9
+
 // The stress utility is intended for catching of episodic failures.
 // It runs a given process in parallel in a loop and collects any failures.
 // Usage: