exec: introduce target_words_bigendian() helper

exec: introduce target_words_bigendian() helper

We currently have a virtio_is_big_endian() helper that provides the target
endianness to the virtio code. As of today, the helper returns a fixed
compile-time value. Of course, this will have to change if we want to
support target endianness changes at run-time.

Let's move the TARGET_WORDS_BIGENDIAN bits out to a new helper and have
virtio_is_big_endian() implemented on top of it.

This patch doesn't change any functionality.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>