Add support for big endian architecture (s390x)

Noticed that some of the Cassandra tests are failing on big endian architectures (s390x). Please find the attached code patch which corrects the data handling for big endian architecture. It also corrects the byte ordering when dealing with sun.misc.Unsafe methods. After the patch following tests are passing for s390x which were failing earlier:

TTLTest
ScrubTest
LegacySSTableTest
SSTableExportSchemaLoadingTest
SSTableMetadataViewerTest
StandaloneUpgraderOnSStablesTest
StandaloneVerifierOnSSTablesTest

The code change ensures that Cassandra for little endian architectures remain unaffected.