cpu: add //go:build line to cpu_gccgo_x86.c

Change-Id: I597a83c767bf597f4d5c2ffdd7097f3cd6781f84
Reviewed-on: https://go-review.googlesource.com/c/sys/+/460158
Auto-Submit: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: tklauser

cpu/cpu_gccgo_x86.c

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build (386 || amd64 || amd64p32) && gccgo
 // +build 386 amd64 amd64p32
 // +build gccgo