Implemement ORC-based "ExecutionYengine"

[dtzWill]

ORC is the newest and most powerful LLVM JIT interface, with the potential for many benefits
through its collection of Layers and the functionality and flexibility they provide.

In particular:

• Lazy compilation opportunities
• (Partial?) weak symbol resolution support
• Better control and default behavior for symbol resolution

The LLVM JIT tutorials look like an excellent place to start:

http://www.llvm.org/docs/tutorial/BuildingAJIT1.html

[dtzWill]

A number of our musl patches only exist because of problems with "weak aliases", not sure if ORC already has ways of dealing with that but it at least exposes the symbol resolution process so that we can do something about it if we wanted :).

[dtzWill]

@tarunraj16 is working on this! 😁