Add ASV benchmarks for MCX gates

What should we add?

Following the significant performance and quality improvement of the MCX synthesis (see details in https://github.com/Qiskit/qiskit/issues/13930) we would like to add the MCX synthesis benchmarks to the ASV benchmarks in https://github.com/Qiskit/qiskit/tree/main/test/benchmarks. Some specific benchmarks are suggested below.