Fix: define old_sigaction as compat_old_sigaction in x86-32 compat

Fix: define old_sigaction as compat_old_sigaction in x86-32 compat

In a follow-up patch which updates the instrumentation for x86-32, 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syscall that can be disabled (it was superseded by rt_sigaction a long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: I633c80e03947a81d1b5c93f8f95a61feb7c34884