[FR] Add z/OS case to ThreadCPUUsage to avoid error

[abhina-sree]

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We are hitting the following error "Per-thread timing is not available on your system." because we do not support CLOCK_THREAD_CPUTIME_ID.

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I will implement a solution similar to SOLARIS that calls ProcessCPUUsage instead

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[LebedevRI]

This has been fixed by https://github.com/google/benchmark/pull/1719.