[POWERPC] Add CHECK_FULL_REGS in several places in ptrace code

This restores the CHECK_FULL_REGS sanity check to every place that can
access the nonvolatile GPRs for ptrace.  This is already done for
native-bitwidth PTRACE_PEEKUSR, but was omitted for many other cases
(32-bit ptrace, PTRACE_GETREGS, etc.); I think there may have been more
uniform checks before that were lost in the recent cleanup of GETREGS et al.

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