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GH-44767: [C++] Fix Float16.To{Little,Big}Endian on big endian machines (#44768)

### Rationale for this change

See issue.

### What changes are included in this PR?

For `ToLittleEndian`/`ToBigEndian`, the result should always be in the specified endianness, not depend on host order.

In the test, instead of casting the `uint8_t` data into a `uint16_t` (with unspecified endianness handling), compare the bytes directly in their expected orders.

### Are these changes tested?

Tested on little-endian, still building for big-endian.

### Are there any user-facing changes?

Fixes #44767
* GitHub Issue: #44767

Authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>