debuginfo: Make variables captured in unboxed closures available in debuginfo.
librustrt: stack_overflow support mipsel linux
auto merge of #19363 : michaelwoerister/rust/support-unboxed-closures, r=alexcrichton

This PR lets `rustc` generate debuginfo for variables captured by unboxed closures. 

Fixes #19356

@nikomatsakis This PR will probably conflict with #19338. If this gets merged before, you should be able to just leave the test case as it is (maybe remove the `#![feature(unboxed_closures)]` directive).
auto merge of #19366 : liigo/rust/mipsel-linux, r=alexcrichton

Since #19076 was merged, I believe mipsel + linux maybe need add to the list here, too.