eal: pause while busy-waiting for lcore slave

Instead of simply busy-waiting for slave in rte_eal_wait_lcore()
do rte_pause(). This will give power savings.

This also fixes warning -Wempty-body seen with armv8a clang
compilation.

Suggested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Signed-off-by: Ashwin Sekhar T K <ashwin.sekhar@caviumnetworks.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>