runtime: fix vet warning about +build rule

cgo_ppc64x.go:7: +build comment must appear before package clause and be followed by a blank line

Change-Id: Ib6dedddae70cc75dc3f137eb37ea338a64f8b595
Reviewed-on: https://go-review.googlesource.com/16835
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/cgo_ppc64x.go

@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package runtime
-
 // +build ppc64 ppc64le
 
+package runtime
+
 // crosscall_ppc64 calls into the runtime to set up the registers the
 // Go runtime expects and so the symbol it calls needs to be exported
 // for external linking to work.