cmd/dist: fix bootstrapping with gccgo on 386

We forgot to add the !gccgo tag to cpuid_386.s.

Change-Id: I2de2ed92ac9686c9365cb37cd29121fa98c2bf37
Reviewed-on: https://go-review.googlesource.com/8960Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/cpuid_386.s

@@ -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 !gccgo
+
 TEXT ·cpuid(SB),$0-8
 	MOVL ax+4(FP), AX
 	CPUID