Fix 4 failing test cases for AIX

[KamathForAIX]

Summary

Platform: PowerPC

OS: AIX

Faiss version: Master branch

Installed from: compiled by myself

Faiss compilation options: Standard instructions using cmake

Reproduction instructions

Run the test cases in AIX. 4 of them will fail. They are testCppcontribUintreader.Test8bit , testCppcontribUintreader.Test10bit, testCppcontribUintreader.Test12bit and testCppcontribUintreader.Test16bit

This happened because the Uint8Reader, Uint10Reader, Uint12Reader and Uint16Reader containers are designed for little endian format in faiss/cppcontrib/detail/UintReader.h.

Handling big endian format has fixed these issues in AIX. Kindly let me know what you think.

[alexanderguzhva]

@KamathForAIX I wrote these utilities for x86 needs, indeed. Also, they are not a part of the 'core' Faiss, they belong to optimized vector codecs & decodecs for PQ / IVFPQ part, which is placed in cppcontrib directory. So, you may keep it as is, unless you plan to use these related components extensively.
Nevertheless, the code changes are appreciated.
I might update the code if I have some free time.

[KamathForAIX]

@alexanderguzhva Thank you for your response. I understand. It will be nice if AIX also passes all test cases, like other platforms.

Do let me know what you think whenever you are available for the same. Sure, I will wait for your response.

[mdouze]

Let's follow up on the PR