x/sys/cpu: TestPPC64minimalFeatures consistently failing on aix-ppc64

[bcmills]

2021-05-07T01:43:57-30e306a/aix-ppc64
2021-05-03T17:37:54-0981d60/aix-ppc64
2021-05-03T08:07:04-8803ae5/aix-ppc64
2021-05-03T07:37:44-b677753/aix-ppc64
2021-05-02T18:08:10-71e4cd6/aix-ppc64
2021-04-26T23:07:00-d19ff85/aix-ppc64
2021-04-21T22:16:51-33663a6/aix-ppc64
2021-04-20T20:58:09-ac73e9f/aix-ppc64
2021-04-20T07:25:15-93ed5bc/aix-ppc64
2021-04-19T17:01:43-37df388/aix-ppc64
2021-04-15T04:56:47-66c3f26/aix-ppc64
2021-04-14T05:50:47-fe65e33/aix-ppc64
2021-04-12T22:04:55-f1c623a/aix-ppc64
2021-04-12T22:03:33-46babdb/aix-ppc64
2021-04-03T16:11:42-5e06dd2/aix-ppc64
2021-04-02T19:21:33-7001323/aix-ppc64
2021-03-31T17:51:45-43e1dd7/aix-ppc64
2021-03-30T21:06:17-4fbd30e/aix-ppc64
2021-03-30T21:06:03-9b9b09a/aix-ppc64
2021-03-26T22:08:04-49726bf/aix-ppc64
2021-03-24T05:16:08-47abb65/aix-ppc64

CC @tklauser @Helflym @ianlancetaylor

[bcmills]

See previously #30732.

[laboger]

The aix builder machine is now Power9.
The way AIX is setting these flags is different from Linux.
On Linux the HWCAP flags for Power8 and Power9 are both true on a Power9 system since they are based on the ISA level and and the ISA for Power9 is a superset of Power8. Looks like AIX sets one or the other.
@Helflym

[martisch]

I think we should fix this by aligning the AIX behaviour to the Linux behaviour. When Power9 is queried and returns true on aix we should set both Power8 and Power9 cpu feature bools.

[gopherbot]

Change https://golang.org/cl/318549 mentions this issue: cpu: set PPC64.IsPOWER8 for Power9 on aix