[POWERPC] Add user_regset compat support

This extends task_user_regset_view CONFIG_PPC64 with support for the
32-bit view of register state, compatible with what a CONFIG_PPC32
kernel provides.  This will enable generic machine-independent code to
access user-mode threads' registers for debugging and dumping.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>