Added support for dynamic loading of variables

[pradt2]

This expands support for dynamic loading to support dynamically loaded variables.

This PR addresses my own grievance - issue #2113. It plays nicely with --dynamic-link-require-all. I tried to mirror the approach used to support dynamically loaded functions.

It's my first contribution to this project, all feedback welcome.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[pradt2]

Hey, thanks for looking at this.

Sure, I can try to add some tests. Would you rather have me add them here, or merge this and open a separate PR?

@emilio

[emilio]

Let's add them here, so the code doesn't land untested.

[pradt2]

@emilio Sorry it took this long, I've added some tests. Let me know if there's anything outstanding still

[bors-servo]

:umbrella: The latest upstream changes (presumably 310f7f877042f0858cd01fb1c326a41d23bd821c) made this pull request unmergeable. Please resolve the merge conflicts.

[pvdrz]

@pradt2 do you have time to rebase this? I can do it if that's not the case

[pradt2]

@pvdrz done

[bors-servo]

:umbrella: The latest upstream changes (presumably #2284) made this pull request unmergeable. Please resolve the merge conflicts.