RFC: linker placement attribute proposed implementation

This is the implementation of feature in https://github.com/rust-lang/rfcs/pull/44, it allows to use the following code:

```rust
#[unsafe_override_address]
static t: T = 123;
```

that will create a t pointing to 123 address in memory.

I also have an alternative implementation that uses `&'static T` and requires less patching, not sure which one is better.