cpu: set PPC64.IsPOWER8 for Power9 on aix

This is in line with the behavior on linux.

Fixes golang/go#46040

Change-Id: I01ba1896d879425d12096ae67250d7e60beb7a99
Reviewed-on: https://go-review.googlesource.com/c/sys/+/318549
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Martin Möhrmann <moehrmann@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
diff --git a/cpu/cpu.go b/cpu/cpu.go
index f77701f..abbec2d 100644
--- a/cpu/cpu.go
+++ b/cpu/cpu.go
@@ -154,14 +154,13 @@
 // For ppc64/ppc64le, it is safe to check only for ISA level starting on ISA v3.00,
 // since there are no optional categories. There are some exceptions that also
 // require kernel support to work (DARN, SCV), so there are feature bits for
-// those as well. The minimum processor requirement is POWER8 (ISA 2.07).
-// The struct is padded to avoid false sharing.
+// those as well. The struct is padded to avoid false sharing.
 var PPC64 struct {
 	_        CacheLinePad
 	HasDARN  bool // Hardware random number generator (requires kernel enablement)
 	HasSCV   bool // Syscall vectored (requires kernel enablement)
 	IsPOWER8 bool // ISA v2.07 (POWER8)
-	IsPOWER9 bool // ISA v3.00 (POWER9)
+	IsPOWER9 bool // ISA v3.00 (POWER9), implies IsPOWER8
 	_        CacheLinePad
diff --git a/cpu/cpu_aix.go b/cpu/cpu_aix.go
index 28b5216..8aaeef5 100644
--- a/cpu/cpu_aix.go
+++ b/cpu/cpu_aix.go
@@ -20,6 +20,7 @@
 		PPC64.IsPOWER8 = true
 	if impl&_IMPL_POWER9 != 0 {
+		PPC64.IsPOWER8 = true
 		PPC64.IsPOWER9 = true