[POWERPC] Use user_regset accessors for GPRs

This implements user_regset-style accessors for the powerpc general
registers.  In the future these functions will be the only place that
needs to understand the user_regset layout (core dump format) and how
it maps to the internal representation of user thread state.

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