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 <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

cpu/cpu_gccgo_x86.c

diff --git a/cpu/cpu_gccgo_x86.c b/cpu/cpu_gccgo_x86.c
index a4605e6..6cc7310 100644
--- a/cpu/cpu_gccgo_x86.c
+++ b/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