io: declare __cpu_is_be in generic code

To use the swap byte transformations in big endian architectures,
we need to declare __cpu_is_be in the generic code.
Let's move it from the ppc code to the generic code.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Message-Id: <20211118145406.340503-1-pmorel@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>