support nonlazybind for extern declarations

[andrewrk]

http://llvm.org/docs/LangRef.html#function-attributes

This attribute suppresses lazy symbol binding for the function. This may make calls to the function faster, at the cost of extra program startup time if the function is not called during program startup.

Currently the default is lazy binding. At the very least we should have the ability to specify nonlazybind for extern declarations. Some other things to explore:

• Ability to tag a scope as wanting all functions called within to be marked nonlazybind.
• Static control flow analysis - if a function is provably called from the main entry point, mark it nonlazybind