auto merge of #12556 : alexcrichton/rust/weak-linkage, r=brson

It is often convenient to have forms of weak linkage or other various types of
linkage. Sadly, just using these flavors of linkage are not compatible with
Rust's typesystem and how it considers some pointers to be non-null.

As a compromise, this commit adds support for weak linkage to external symbols,
but it requires that this is only placed on extern statics of type `*T`.
Codegen-wise, we get translations like:

```rust
    // rust code
    extern {
        #[linkage = "extern_weak"]
        static foo: *i32;
    }

    // generated IR
    @foo = extern_weak global i32
    @_some_internal_symbol = internal global *i32 @foo
```

All references to the rust value of `foo` then reference `_some_internal_symbol`
instead of the symbol `_foo` itself. This allows us to guarantee that the
address of `foo` will never be null while the value may sometimes be null.

An example was implemented in `std::rt::thread` to determine if
`__pthread_get_minstack()` is available at runtime, and a test is checked in to
use it for a static value as well. Function pointers a little odd because you
still need to transmute the pointer value to a function pointer, but it's
thankfully better than not having this capability at all.

Thanks to @bnoordhuis for the original patch, most of this work is still his!