support aggregation table

Aggregation table is a percpu table, so each cpu update
its own table, no table lock need to accquire.

Currently we don't change compiler to support aggregation,
so library call is needed, see example as below:

	s = aggr_table()

	trace "*:*" {
		s[argname] = count()
	}

	trace_end {
		histogram(s)
	}

This approach will reduce ktap overhead heavily, also
reduce lock contention, note that we cannot use this
aggregation in everywhere, for example, you can not
use aggregation for thread-local associative array.

Four aggregation functions was implemented at present:

	s[KEY] = count()
	s[KEY] = max(N)
	s[KEY] = min(N)
	s[KEY] = avg(N)

you have to use aggr_table() library function to allocate
a new aggregation table, in future, perhaps we can use @
to repesent aggregation table like below:
	@s = {}

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