ppc: Fix size of ppc64 xer register

ppc: Fix size of ppc64 xer register

The normal gdb definition of the XER registers is only 32 bit,
and that's what the current version of power64-core.xml also
says (seems copied from gdb's).  But qemu's idea of the XER register
is target_ulong (in CPUPPCState, ppc_gdb_register_len and
ppc_cpu_gdb_read_register)

That mismatch leads to the following message when attaching
with gdb:

  Truncated register 32 in remote 'g' packet

(and following on that qemu stops responding).  The simple fix is
to say the truth in the .xml file.  But the better fix is to
actually make it 32bit on the wire, as old gdbs don't support
XML files for describing registers.  Also the XER state in qemu
doesn't seem to use the high 32 bits, so sending it off to gdb
doesn't seem worthwhile.

Signed-off-by: Michael Matz <matz@suse.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>