extern(C) in druntime

[Sean Kelly via Digitalmars-d]

Also, eliminating compile time dependencies between namespaces. 
Everything can import core.*, but nothing outside rt is allowed 
to import rt modules and nothing outside gc is allowed to import 
gc modules. This allows us to theoretically choose the compiler 
runtime and GC at link time and also allows us not to put this 
code in the import tree when distributing druntime.  In short, 
extern (C) is often used because you can't forward declare extern 
(D) functions.  It doesn't mean anything about the language used 
by the function.