[SPARK-18963] o.a.s.unsafe.types.UTF8StringSuite.writeToOutputStreamIntArray test

fails on big endian. Only change byte order on little endian

## What changes were proposed in this pull request?

Fix test to only change byte order on LE platforms

## How was this patch tested?

Test run on Big Endian and Little Endian platforms

Author: Pete Robbins <robbinspg@gmail.com>

Closes #16375 from robbinspg/SPARK-18963.