Rectify in-memory buffer model loading for s390x
Hi @Ferev  Can you please review this PR ? Thank you very much!
cc @gbaned  Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you! Hi @Ferev Can you please review this PR ? Thank you!
After introducing [commit](https://github.com/tensorflow/tensorflow/commit/38a44709efd6e51421a58bfb7df32cf9f7af8fff) to add in-memory model support for mini-benchmark, below test cases started to fail on s390x(BE machines) :
```
//tensorflow/lite/experimental/acceleration/mini_benchmark:validator_runner_impl_test
//tensorflow/lite/experimental/acceleration/mini_benchmark:blocking_validator_runner_test
```
Reason for the failures is because TFLite buffers in buffer model are being loaded in LE format.
After this PR, buffer model will be loading TFLite buffers in BE format for BE plaforms, rectifying above mentioned test cases for s390x. 

These changes will not cause any regression on LE/BE platforms.