[AIX]test failure fix using gtest-1.15.0 for AIX (#21497)
### Description
Local CI setup for AIX reported tests failure after the gtest 1.15.0
upgrade.
### Motivation and Context
Below tests failure is observed after gtest upgrade.
The following tests FAILED:
1 - onnxruntime_test_all (ILLEGAL)
7 - onnxruntime_logging_apis_test (Subprocess aborted)
To fix this, I am enabling pthread support under gtest. This was
disabled with previous version of gtest for some reason.
Now by enabling this, above tests are getting passed with gtest 1.15.0.
