For now, the most efficient approaches in practice seems to follow the principle of symbolic execution. This technique, originally developed to automatically build-up a set of test cases based on a given source code, has been recently used in binary analysis to discover and (partially) recover the CFG of the analyzed binary program.
Most of the time, if you want to deal with several assembly languages, you need to use an intermediate representation for the programs. For now, there have been a lot of these generic models and none did really got the supremacy over the others. Yet, the most populars seems to be the intermediate representation of LLVM (many tools use it), the second most popular seems to be VEX, the intermediate representation used in Valgrind (as used in Angr). The one from QEMU or the language RREIL might also be an option, but they are less commonly used. But, most of the projects come with their own intermediate representation.
Then, what you need to build up a CFG-recovery program based on symbolic execution are the following modules:
Loader: This module is in charge to take your binary program (it may be an executable or a library) and simulate the work of the native loader to build a realistic memory image of what you get after the loading process has been achieved.
Decoder: This module is in charge of translating the native assembly found while symbolically executing the program into your own intermediate representation. It is a quite lengthy (and painful) piece of code to implement! So, be prepared to suffer while doing this.
Symbolic execution engine: Usually based on an SMT-solver using the logic QF_AUFBV (Quantifier Free / Arrays / Uninterpreted Function / Bitvectors), the symbolic execution engine can really be a performance bottleneck if you code it naively because the recovery of the CFG will use it a lot. Here, having a good formula simplification (or slicing) module is really the key.
Apart from these three modules, you might also improve your tools by adding more advanced analysis and start coding an abstract interpretation framework that can be added on the top of your intermediate representation, just to have chance to unveil some parts of the CFG that cannot be discovered just through the power of the SMT-solvers.
Also, performance is really the key to make your tools really usable. So, be able to capture the exact scope of a variable or the ability to detect a function or a module/object in the binary code helps a lot to reduce the size of the code you have to consider at once.
Now, I could give you a lot of pointers and articles about this topic, but I lack a bit of time. I might come back to finish writing an extensive list later on, but the general idea has been, hopefully, given above. Hope this might help you.