cpu: don't depend on the golang.org/x/sys/unix package for AIX

gccgo support can happen in a future CL.

Updates golang/go#32102

Change-Id: Ic9e8d7b3e413079d277bdba565551845a2b78121
Reviewed-on: https://go-review.googlesource.com/c/sys/+/179178
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

cpu/cpu_aix_ppc64.go

@@ -6,8 +6,6 @@
 
 package cpu
 
-import "golang.org/x/sys/unix"
-
 const cacheLineSize = 128
 
 const (
@@ -18,7 +16,7 @@ const (
 )
 
 func init() {
-	impl := unix.Getsystemcfg(_SC_IMPL)
+	impl := getsystemcfg(_SC_IMPL)
 	if impl&_IMPL_POWER8 != 0 {
 		PPC64.IsPOWER8 = true
 	}
@@ -28,3 +26,9 @@ func init() {
 
 	Initialized = true
 }
+
+func getsystemcfg(label int) (n uint64) {
+	r0, _ := callgetsystemcfg(label)
+	n = uint64(r0)
+	return
+}