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ARROW-9681: [Java] Fix test failures of Arrow Memory - Core on big-endian platform

This PR fixes two test failures on BE platform by taking care of big-endian in `setBytes()` and `memcpy()`.

```
% mvn -B -Drat.skip=true -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn -Dflatc.download.skip=true -rf :arrow-memory-core test
...
[ERROR]   TestArrowBuf.testSetBytesBigEndian:145 arrays first differed at element [0]; expected:<0> but was:<7>
[ERROR]   TestByteFunctionHelpers.testStringCompare Multiple Failures (2 failures)
	expected:<-1> but was:<1>
...
[ERROR] Tests run: 31, Failures: 2, Errors: 0, Skipped: 0
```

Closes #7923 from kiszk/ARROW-9681

Authored-by: Kazuaki Ishizaki <ishizaki@jp.ibm.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>