internal/syscall/unix: restore ppc build tag

It was accidentally lost in the 1.15rc1 merge.

Fixes PR go/96567

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/247843

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c512af85eb8c75a759b5e4fc6b72041fe09b75f1
+e08f1d7d1bc14c0a29eb9ee17980f14fa2397239
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/internal/syscall/unix/sysnum_linux_ppc64x.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build ppc64 ppc64le
+// +build ppc ppc64 ppc64le
 
 package unix