#[link_section] (and maybe more) should be applied to the trampoline

[jonas-schievink]

In some not-yet-open-sourced code, we have used #[link_section] to place 2 functions, one of which is an #[exception] handler, in the same section so they're closer together, in order to link correctly (one of them calls the other using inline assembly with limited jump range).

With #205, this code broke because #[link_section] does not get applied to the trampoline, and the compiler decides to inline the function with the #[link_section] attribute into the trampoline. Whether this inlining decision makes sense is another question, but this still broke code that one should reasonably expect to work.

There's a few ways to fix this or work around the issue:

• Put #[inline(never)] on the user code (this is what I've done for now, and it fixes linkage, but wastes code space because this really should be inlined)
• Declare the inlining as a rustc bug, report it, and forget about it.
• Have the #[exception] macro (and the others) duplicate some attributes and apply them to both function in the output.

The last solution would be ideal, since it fixes the user-visible regression in behavior without preventing the inlining, and makes attributes on #[exception] functions behave more similarly to how they behave on regular, single functions. I don't know which other linkage-related attributes need to be duplicated though.