MIPS: Clean up compat_siginfo_t

MIPS: Clean up compat_siginfo_t

While mips can't use the generic compat_siginfo_t directly because
its si_code and si_errno are inverted, we can still make it as
close to the generic version as possible. This makes it easier
to update when new members are added to siginfo_t.

The main changes are adding a missing _sigsys union member and
eliminating the unused _irix_sigchld one.

Signed-off-by: Amanieu d'Antras <amanieu@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11455/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>