Generic functions can't call external C functions

[o11c]

Code of the general form:

extern { fn foo(); }

pub fn call_foo<T>() { foo(); }

will not allow other crates to use call_foo, because they don't link directly against the library that foo is defined in.

The solution is probably to emit wrappers or aliases for the C function in the dylib itself.
(I am not concerned about rlibs.)

A full testcase is at: https://gist.github.com/o11c/3cedbabc14adcb3e5254 (tested on Linux)

[alexcrichton]

Closing as a dupe of #15460