Initial implementation of @extern builtin

[LemonBoy]

I heard from the grapevine somebody needs weak extern symbols.

MVP based on our previous discussion & failed attempts.

[andrewrk]

I'm actually a little bit further along than this in a local branch - my strategy is to append to a list and then iterate over it in do_code_gen. It works for things in a comptime scope too

[andrewrk]

ok my code is not at a point where it's working though so I'm tempted to merge this as is since we have to re-implement it in stage2 anyway

would this unblock #4503 as-is?

[LemonBoy]

I'm actually a little bit further along than this in a local branch

The only missing thing is documentation and (hopefully) a few changes to let the parser allow @extern, right now it rejects that with a syntax error because extern is a keyword (?).

Edit: The parser part was really easy.

would this unblock #4503 as-is?

AFAIR I needed this to check for _DYNAMIC being defined or not. Yes, this should be enough.

[suirad]

How does this compare with @export(fnname, "out name", .Weak)?

When I was using the above I was hoping it would work like the extern.

[LemonBoy]

How does this compare with @export(fnname, "out name", .Weak)?

That exports a weak symbol, extern lets you import weak (or strong) symbols.