[POWERPC] Add compat handler for PTRACE_GETSIGINFO

Current versions of gdb require a working implementation of
PTRACE_GETSIGINFO for proper watchpoint support.  Since struct siginfo
contains pointers it must be converted when passed to a 32-bit debugger.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>