Auto merge of #144678 - jdonszelmann:no-mangle-extern, r=bjorn3

Make no_mangle on foreign items explicit instead of implicit

for a followup PR I'm working on I need some foreign items to mangle. I could add a new attribute: `no_no_mangle` or something silly like that but by explicitly putting `no_mangle` in the codegen fn attrs of foreign items we can default it to `no_mangle` and then easily remove it when we don't want it.

I guess you'd know about this r? `@bjorn3.` Shouldn't be too hard to review :)

Builds on rust-lang/rust#144655 which should merge first.

Author: bors