powerpc: Update compat_arch_ptrace

Update compat_arch_ptrace to follow recent changes in
PTRACE_GET_DEBUGREG and the addition of
PPC_PTRACE_{GETHWDBGINFO|{SET|DEL}HWDEBUG}.  The latter three can be
forwarded to arch_ptrace unchanged.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>