Fix: remove ARM set_tls system call override

We'll need to find a better way to instrument ARM-specific system calls
located at a far offset from the standard systems calls. A 16MB
lttng-modules kernel module is really not acceptable.

Removing this instrumentation for now. sys_set_tls will appear as
sys_unknown.

Fixes #472

CC: Ryan Kyser <Ryan.Kyser@jci.com>
Ref: http://lists.lttng.org/pipermail/lttng-dev/2013-April/019990.html
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>