Reland "[starnix] Implement getcpu()."

This is a reland of commit 092bff7f9ea509e7f5fe9bb25a300917cb704dcb

Skips LTP's getcpu01 test which will only work some of the
time without CPU affinity support.

Multiply: ltp_tests_bundle_g
Original change's description:
> [starnix] Implement getcpu().
>
> Bug: b/325643815
> Fixed: b/297305396
> Change-Id: Icc141be4df2ffc5cff6e9d45ff9ea478eabb1ffa
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/993153
> Reviewed-by: James Robinson <jamesr@google.com>
> Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
> Fuchsia-Auto-Submit: Adam Perry <adamperry@google.com>

Bug: b/325643815
Change-Id: I01f12e25172387ad17935b02a311b41a0fb249ca
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/995497
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: James Robinson <jamesr@google.com>
Fuchsia-Auto-Submit: Adam Perry <adamperry@google.com>