Elastic Load Balancing gives you a CNAME as an endpoint. Here's an example:
$ host www.foursquare.com
www.foursquare.com is an alias for star-foursquare-com-367497865.us-east-1.elb.amazonaws.com.
As you can see, ELB works by providing a
.wherever.elb.amazonaws.com domain name, with a low TTL. Amazon will then return you different DNS values for the ELB based on where you are coming from, and what backend servers are up. (I wrote a blog post on this topic once which I encourage you to read if you want more detail.)
So, you can make
site.abc.com or even
*.abc.com a CNAME to your
.wherever.elb.amazonaws.com address, and the name will be forwarded to the instances behind it as you would expect.
aaa.com is trickier, as the zone apex (root name) cannot generally be a CNAME. The previous workaround was to point aaa.com to a single EC2 instance on an elastic IP, which you would re-map in the case of failure; however, Amazon recently released an addition to their Route 53 DNS service where they will set keep the DNS records for aaa.com in sync with what your ELB CNAME will return.