internal/cpu: set PPC64.IsPOWER8
internal/cpu: set PPC64.IsPOWER8

This should always be true, but use the HWCAP2 bit anyways.

Change-Id: Ib164cf05b4c9f0c509f41b7eb339ef32fb63e384
Reviewed-on: https://go-review.googlesource.com/c/go/+/389894
Trust: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>
Run-TryBot: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go