target-ppc: ppc64 can be either endian

POWER7, POWER7+ and POWER8 families use the ILE bit of the LPCR special
purpose register to decide the endianness to use when entering interrupt
handlers. When running a linux guest, this provides a hint of the
endianness used by the kernel. From a qemu point of view, the information
is needed for legacy virtio support and possibly crash dump support as well.

Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
target-ppc: enable virtio endian ambivalent support

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>