runtime: don't build semasleep_test on Plan 9

CL 135015 added TestSpuriousWakeupsNeverHangSemasleep.
However, this test is failing on Plan 9 because
syscall.SIGIO is not defined.

This change excludes semasleep_test.go on Plan 9

Fixes #27662.

Change-Id: I52f9f0fe9ec3c70da5d2f586a95debbc1fe568a1
Reviewed-on: https://go-review.googlesource.com/135315
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/semasleep_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//+build !nacl,!windows,!js
+//+build !nacl,!plan9,!windows,!js
 
 package runtime_test