x86_64 ia32 ptrace: use compat_ptrace_request for siginfo

This removes the special-case handling for PTRACE_GETSIGINFO
and PTRACE_SETSIGINFO from x86_64's sys32_ptrace.  The generic
compat_ptrace_request code handles these.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>