CTF writer: restore native byte order is the CPU's native byte order

CTF writer: restore native byte order is the CPU's native byte order

To remain compatible with the first experiments of CTF writer, let's
restore this legacy behaviour in which a native byte order means the
native byte order of the CPU running the Babeltrace library.

This only applies to bt_ctf_writer_set_byte_order() when you pass
BT_CTF_BYTE_ORDER_NATIVE: bt_ctf_trace_set_native_byte_order() does
not accept BT_CTF_BYTE_ORDER_NATIVE. Thus, when you create a CTF writer
object with bt_ctf_writer_create(), it sets its trace's native byte
order to the CPU's native byte order.

perf is a CTF writer user known to not call
bt_ctf_writer_set_byte_order() manually: in this case the CTF IR trace
would be invalid because its native byte order would still be set to
BT_CTF_BYTE_ORDER_NATIVE.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>