gdb-xml: fix hacks in powerpc register numbering

The powerpc xml files contained a hack--an empty, non-existent
register--for getting the register numbers to line up for
newer (XML-aware) and older (non-XML-aware) GDB.  While this hack worked
in some cases, it didn't work in all cases, notably when the user used
`finish' or `continue': GDB would attempt to read the non-existent
register and QEMU would complain.

This patch fixes things up properly.  Instead of inserting a fake
register, we explicitly declare the floating-point and SPE registers to
start at 71.  This action accomplishes the same thing as the nasty hack,
except that now GDB never tries to fetch the non-existant register 70.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
