x/sys/cpu: TestPPC64minimalFeatures fails on aix/ppc64

[tklauser]

=== RUN   TestAMD64minimalFeatures
--- PASS: TestAMD64minimalFeatures (0.00s)
=== RUN   TestAVX2hasAVX
--- PASS: TestAVX2hasAVX (0.00s)
=== RUN   TestARM64minimalFeatures
--- PASS: TestARM64minimalFeatures (0.00s)
=== RUN   TestPPC64minimalFeatures
--- FAIL: TestPPC64minimalFeatures (0.00s)
    cpu_test.go:54: IsPOWER8 expected true, got false
=== RUN   TestS390X
--- PASS: TestS390X (0.00s)
FAIL
FAIL	golang.org/x/sys/cpu	0.017s

https://build.golang.org/log/09cf10dbd292a267aae1234be2b2d5c332622d82

We can detect cpu.IsPower8 and cpu.IsPower9 using the getsystemcfg syscall as in package runtime.

/cc @Helflym

[gopherbot]

Change https://golang.org/cl/166465 mentions this issue: unix: add Getsystemcfg on aix

[gopherbot]

Change https://golang.org/cl/166466 mentions this issue: cpu: use unix.Getsystemcfg to detect POWER8/POWER9