Hook into GCC's -finstrument-functions to get the compiler to talk to DR

_From [peter.goodman](https://code.google.com/u/peter.goodman/) on August 05, 2013 16:12:27_

What is the problem to solve? Why is it important? Provide some context for those unfamiliar with the details of the system. This is a solution looking for a problem. The idea is meant to be an intermediate step between having static and dynamic instrumentation work together. GCC already supports limited function instrumentation, using the `-finstrument-functions` flag. With this flag, GCC will add in invocations to two user-defined functions. These calls indicate the begin/end of procedures, and so can be used to disambiguate things like tail-calls. What are the possible approaches to solving the problem? Enable GCC's `-finstrument-functions` flag, and have a client explicitly recognise CALLs to the injected functions and replace those calls with something client-specific, or elide the calls entirely. Which approach is being taken and why? No approach is being taken, likely because it's not clear that this approach would solve any real-world problems (yet?). Any interesting details or challenges of the implementation? No.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1244_