Fix a typo in BenchmarkXxhash64CgoVeryShort
Removed Endianness safety in the native (non-appengine) version.

If you require endianness safety build with -tags be.
Endianness safety is only required if you're trying to compare hashes
on different archs.

benchmark                           old ns/op     new ns/op     delta
BenchmarkXxhash32                   1907          1029          -46.04%
BenchmarkXxhash64                   1097          727           -33.73%
BenchmarkXxhash64VeryShort          19.3          16.8          -12.95%
BenchmarkXxhash64MultiWrites        1352          815           -39.72%