On AIX clock_gettime provides CPU time with a resolution of
10ms, which causes the ThreadTicks testcase to fail since at
the 2 instances the CPU time of the thread outputs to 0.
Using AIX's API thread_cputime instead which provides CPU
time with a resolution of 1ns.
The testcase was added as part of https://codereview.chromium.org/1976603005

R=jochen@chromium.org, lpy@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2174003002
Cr-Commit-Position: refs/heads/master@{#38029}