runtime: usleep computes wrong tv_nsec on s390x [1.24 backport]

[gopherbot]

@pmur requested issue #71714 to be considered for backport to the next 1.24 minor release.

@gopherbot please consider this for backport to 1.23 and 1.24. This is a serious regression that has been around for much longer.

[gopherbot]

Change https://go.dev/cl/649375 mentions this issue: [release-branch.go1.24] runtime: fix usleep on s390x/linux

[gopherbot]

Closed by merging CL 649375 (commit bb0e5c2) to release-branch.go1.24.