add gettimeofday_us/num_cpus built-in function and change count function

Now count function support third argument:
	count(tbl, key, n)

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
add sample script: function_time.kp

for get function executing time

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>