Enable secure grpc++/grpc deps and ssl features on s390x
Thanks @sagunb ! Looks like the following error shown in the PR checking logs wasn't caused by the code change in this PR:

```bash
external/llvm-project/llvm/include/llvm/IR/Attributes.h:91:14: fatal error: 'llvm/IR/Attributes.inc' file not found
    #include "llvm/IR/Attributes.inc"
             ^~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

@sagunb @Neilblaze Could you please help me check the status of this PR? Please let me know if there is any change I should make at my end. Thank you very much! Hi @gbaned ,

Could you please help me check the status of this PR?

Thank you very much! > Hi @gbaned ,
> 
> Could you please help me check the status of this PR?
> 
> Thank you very much!

Hi @kun-lu20 This PR is processing internally. Thank you so much! @gbaned Thanks for checking!
Since `boringssl` has not been supported on s390x yet, `@com_github_grpc_grpc//:grpc_unsecure` and `@com_github_grpc_grpc//:grpc++_unsecure` deps used to be chosen when building TensorFlow on s390x.
 
However, we can enable secure grpc++/grpc deps and ssl related features for TensorFlow on s390x via `openssl` by using `TF_SYSTEM_LIBS="boringssl"` flag in bazel commands.
 
This PR aims to enable the above-mentioned features on s390x by removing the specific `grpc_unsecure` and `grpc++_unsecure` deps from BUILD files and adding the `TF_SYSTEM_LIBS="boringssl"` flag as a default option for Bazel during the configuration stage on s390x.
 
After applying the above change, the following test cases would pass on s390x:
```bash
//tensorflow/core/platform/cloud:compute_engine_metadata_client_test
//tensorflow/core/platform/cloud:compute_engine_zone_provider_test
//tensorflow/core/platform/cloud:curl_http_request_test
//tensorflow/core/platform/cloud:gcs_file_system_test
//tensorflow/core/platform/cloud:google_auth_provider_test
//tensorflow/core/platform/cloud:oauth_client_test
```

Fixes https://github.com/tensorflow/tensorflow/issues/51770 .