[POWERPC] Use regset code for PTRACE_*REGS* requests

This replaces all the code for powerpc PTRACE_*REGS* requests with
simple calls to copy_regset_from_user and copy_regset_to_user.  All
the ptrace formats are either the whole corresponding user_regset
format (core dump format) or a leading subset of it, so we can get
rid of all the remaining embedded knowledge of both those layouts
and of the internal data structures they correspond to.  Only the
user_regset accessors need to implement that.

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