internal/cpu: align s390x code in internal/cpu with that in x/sys/cpu

[billotosyr]

Have recently had z/OS support added to x/sys/cpu (fixed #41984). That CL made most of the code common between z/OS and Linux. This issue tracks updating internal/cpu to include all changes from x/sys/cpu, except for zos-specific files (i.e. only the code which will be common).

[gopherbot]

Change https://golang.org/cl/266457 mentions this issue: internal/cpu: align s390x support with x/sys/cpu

[martisch]

FYI: @billotosyr No need to open up issues for every CL if there is no algorithm or big API change or that do not need discussion or decisions.

[odeke-em]

Bill, apologies that we didn’t get to review this for Go1.16. I have tagged some other reviewers for Go1.17, and I am going to punt this issue for this, as well as mark it as a release blocker.

[gopherbot]

This issue is currently labeled as early-in-cycle for Go 1.17.
That time is now, so a friendly reminder to look at it again.

[ianlancetaylor]

Moving to Backlog.