target-ppc: gdbstub: fix altivec registers for little-endian guests

target-ppc: gdbstub: fix altivec registers for little-endian guests

Altivec registers are 128-bit wide. They are stored in memory as two
64-bit values that must be byteswapped when the guest is little-endian.
Let's reuse the ppc_maybe_bswap_register() helper for this.

We also need to fix the ordering of the 64-bit elements according to
the target endianness, for both system and user mode.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>