x/sys/cpu: add ARM64 and PPC64 support

[FiloSottile]

golang/sys@4adea00 added x/sys/cpu, but there is no support for ARM64 and PPC64 feature detection because that will require some form of runtime participation.

See the end of the #24843 thread for context.

Remember that subrepos need to work with the two last Go releases.

[laboger]

@ceseo will look at this.

[gopherbot]

Change https://golang.org/cl/150718 mentions this issue: x/sys: add linux/arm64

[gopherbot]

Change https://golang.org/cl/152938 mentions this issue: cpu: add linux/ppc64x