Add dummy TLS variable to rseq_offset tests

Add dummy TLS variable to rseq_offset tests

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: I1c50640c4f32ea5ef65b0b823e7be75fd92b303d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>