unix: add //go:build lines to assembly files

The zsyscall_darwin*.s files were regenerated using mksyscall.go. The
rest were updated manually, go fmt doesn't support formatting assembly
files.

Change-Id: I5efa23cb988300257de6bba4769e78c49a6382f1
Reviewed-on: https://go-review.googlesource.com/c/sys/+/312411
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/asm_linux_ppc64x.s b/unix/asm_linux_ppc64x.s
index 459a629..19d4989 100644
--- a/unix/asm_linux_ppc64x.s
+++ b/unix/asm_linux_ppc64x.s
@@ -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 linux && (ppc64 || ppc64le) && gc
 // +build linux
 // +build ppc64 ppc64le
 // +build gc