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GH-39782: [C++] Use correct (non-CPU) address of buffer in ExportDeviceArray (#39783)

### Rationale for this change

The Array exporter currently uses `buffer->data()`, which is documented as "The buffer has to be a CPU buffer (`is_cpu()` is true). Otherwise, an assertion may be thrown or a null pointer may be returned.". 
In practice, we indeed return null for non-CPU data. There is only a debug check that asserts the device. 

This means that for non-CPU data the C device interface is currently returning null pointers for the buffers.

### What changes are included in this PR?

Use the buffer's `address()` instead of `data()`

### Are these changes tested?

Not yet

* Closes: #39782

Lead-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>