Add dummy TLS variable to no_syscall test

Add dummy TLS variable to no_syscall test

Ensure the main executable has at least one TLS variable which will be
allocated before the rseq area, making sure the rseq_offset is not 0.  This
allows testing that the rseq_offset variable is properly initialized by
checking it is not 0.

Most toolchains will add at least one main exec TLS variable but it's
currently not the case on RISC-V.

Change-Id: Ie29bcaa1e008e99953a4f19e19f05c8b85dac62d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>