[C++][Compute] Make GroupBy optimizations work on Big Endian architecture

[asfimport]

This is related to ARROW-12010, which introduces optimized group by implementation.
On s390x (big endian architecture) the code hangs in unit tests (https://travis-ci.com/github/apache/arrow/jobs/506281680).
Currently this implementation is disabled on big endian and the alternative, earlier, implementation is used instead, but at some point we will stop maintaining both and switch to a single implementation, which requires fixing this issue.

Reporter: Michal Nowakiewicz / @michalursa

Related issues:

• [C++] Query engine umbrella issue (is a child of)

_{Note: This issue was originally created as ARROW-12830. Please see the migration documentation for further details.}