Fix: define old_sigaction as compat_old_sigaction in arm32 compat

Fix: define old_sigaction as compat_old_sigaction in arm32 compat

In a follow-up patch which updates the instrumentation for arm32, the
instrumentation of the old_sigaction() syscall would fail to build when
targeting a 64-bit kernel.

In that context, old_sigaction becomes compat_old_sigaction which is a
syscall that can be disabled (it was superseded by rt_sigaction a long
time ago).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I3d9e155f7943cc4b768e13ebd25b671e46df9659