[POWERPC] Allow ptrace write to pt_regs trap and orig_r3

This patch allows a ptracer to write to the "trap" and "orig_r3" words
of the pt_regs.

This, along with a subsequent patch to the signal restart code, should
enable gdb to properly handle syscall restarting after executing a separate
function (at least when there's no restart block).

This patch also removes ptrace32.c code toying directly with the registers
and makes it use the ptrace_get/put_reg() accessors for everything so that
the logic for checking what is permitted is in only one place.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>