auto merge of #16904 : inrustwetrust/rust/link-path-order, r=alexcrichton

auto merge of #16904 : inrustwetrust/rust/link-path-order, r=alexcrichton

Issue can be reproduced by the following:
```
$ cat main.rs
fn main() {}
$ rustc -Z print-link-args -Lfoo -Lbar main.rs
```
Run the rustc command a few times and observe that the order of the '-L' 'foo' '-L' 'bar' options randomly changes.

Actually hit this issue in practice on Windows when specifying two -L directories to rustc, one with rust-sdl2 in it and one with the C SDL2.dll. Since Windows file systems aren't case-sensitive, gcc randomly attempted to link against the rust sdl2.dll instead of SDL2.dll if that -L directory happened to come first.

The randomness was due to addl_lib_search_paths being a HashSet. Changed it to a Vec instead which maintains the ordering.
Unsure how to test this though since it is random by nature; suggestions very welcome.