eal: allow empty compile time cpu flags

eal: allow empty compile time cpu flags

When RTE_COMPILE_TIME_CPUFLAGS is empty, the rte_cpu_check_supported()
code breaks with a "comparison is always false due to limited range of
data type".  This is because the compile_time_flags[] array is empty.
Assigning the array dimension to a local variable apparently solves this.

Signed-off-by: Cyril Chemparathy <cchemparathy@ezchip.com>
Signed-off-by: Zhigang Lu <zlu@ezchip.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>