compiletest does not pass `--extern` flags

[nikomatsakis]

In #51456, @qmx and I noticed that compiletest, when using aux-build libraries, simply supplies the compiler with -L instead of using the --extern lib_name=path-to-rlib compiler argument form. This is a shame because the latter form is what cargo actually uses in practice!

We ought to add another comment form to use the latter. Something like:

// aux-build:lib_name=file.rs

or perhaps a different header name.

(We could also retrofit the existing aux-build to use --extern, but it seems useful to be able to test in both modes, actually, so I'd prefer to add something new.)

[eddyb]

I have a branch where I implemented the --extern crate_name form without a path, (which was suggested for Rust 2018) and started using it with // compile-flags:--extern lib_name.

Which way is preferred here?

[qmx]

@eddyb care to link the branch?

[nikomatsakis]

@eddyb my preference would be to be able to test All The Things, I guess? That is, I see value in both. I guess the motivation for --extern though is to make Rust 2018 niceties available to people that are not using cargo (and hence want -L)? (Which makes sense)