config: clean cache line size selection scheme

config: clean cache line size selection scheme

by default, all the targets will be configured with the 64-byte cache line
size, targets which have different cache line size can be overridden
through target specific config file.

Selected ThunderX and power8 as CONFIG_RTE_CACHE_LINE_SIZE=128 targets
based on existing configuration.

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>