cpu: add build tag to cpu_wasm.go

Fixes the build for Go versions before 1.11.

Fixes golang/go#30668

Change-Id: I0dfb2eafcbcbe7e466a3b0b5d20656be3e6f4982
Reviewed-on: https://go-review.googlesource.com/c/sys/+/166137
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cpu/cpu_wasm.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build wasm
+
 package cpu
 
 // We're compiling the cpu package for an unknown (software-abstracted) CPU.