The API thread_cputime() is only defined but not yet implemented on IBMi.

Change-Id: I8ea7ff724e749f537b54e75a00d718500807ca8a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1957831Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#65392}