Consider making reachable extern fns #[no_mangle] by default

[alexcrichton]

It's a little annoying to have to spray this attribute everywhere

Chances are if I have an extern function which is reachable that I'm going to want to call it from C or something similar.

Nominating.

[pnkfelix]

Accepted for 1.0, P-backcompat-lang. Also, we want to discuss further in an actual team mtg.

[alexcrichton]

Closing, we decided in today's meeting to not do this.