bpo-34373: fix test_mktime and test_pthread_getcpuclickid tests on AIX (GH-8726)

* Fix test_mktime on AIX by adding code to get mktime to behave the
  same way as it does on other *nix systems
* Fix test_pthread_getcpuclickid in AIX by adjusting the test case
  expectations when running on AIX in 32-bit mode

Patch by Michael Felt.