[rfc] add callgraph PC annotation to pmcstat

Hi,

This patch adds callgraph annotation to pmcstat via -a. It doesn't
(yet) add line numbers; I'll see if I can easily add that without too
much effort.

This is like (and based on) the -m option but the -m option only
prints the first entry in the callgraph. It isn't very useful if you
want to see what actually called it and where it called it from, in
order to trace things like "where's that memcpy actually coming from?"

http://people.freebsd.org/~adrian/netflix/20140218-pmcstat-callgraph-annotate.diff

Thanks!



-a