[POWERPC] Add user_regset_view definitions

This provides the task_user_regset_view entry point and support for
all the native-mode (64 on CONFIG_PPC64, 32 on CONFIG_PPC32) thread
register state.  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>