eal: add nanosleep based delay function

eal: add nanosleep based delay function

Add a new rte_delay_us_sleep() function that uses nanosleep().
This function can be used by applications to not implement
their own nanosleep() based callback and by internal DPDK
code if CPU non-blocking delay needed.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>