[AIX]test failure fix using gtest-1.15.0 for AIX

[ranjitshs]

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.

[ranjitshs]

@snnn
As mentioned in #21410 , this PR contains the AIX test failure fix with gtest 1.15.0.
Please trigger the relevant CI pipeline and let me know if I need to create a corresponding issue.

@tianleiwu FYI.

[snnn]

/azp run Big Models, Linux Android Emulator QNN CI Pipeline, Linux CPU CI Pipeline, Linux CPU Minimal Build E2E CI Pipeline, Linux GPU CI Pipeline, Linux GPU TensorRT CI Pipeline, Linux OpenVINO CI Pipeline, Linux QNN CI Pipeline, MacOS CI Pipeline

[snnn]

/azp run Windows ARM64 QNN CI Pipeline, Windows CPU CI Pipeline, Windows GPU CI Pipeline, Windows GPU TensorRT CI Pipeline, Windows x64 QNN CI Pipeline, onnxruntime-binary-size-checks-ci-pipeline, orttraining-linux-ci-pipeline, orttraining-linux-gpu-ci-pipeline, orttraining-ortmodule-distributed

[azure-pipelines]

Azure Pipelines successfully started running 9 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 9 pipeline(s).

[ranjitshs]

@snnn @tianleiwu
if no comments on this PR, can it get merge ? Please merge it.