[POWERPC] powerpc: ptrace can set DABR on both 32 and 64 bits

Allow ptrace to set dabr in the thread structure for both 32 and 64 bits,
though only 64 bits actually uses that field, it's actually defined in both.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>