Add `needs-unwind` and beginning of support for testing `panic=abort` std to compiletest
For the Fuchsia platform we build libstd with `panic=abort` and would like a way to run tests with that enabled. This adds low-level support for this directly to compiletest.
In the future I'd like to add high-level support in rustbuild, e.g. having target-specific flags that allow configuring a panic strategy. (Side note: It would be nice if we could also build multiple configurations for the same target, but I'm getting ahead of myself.)
This plus #84500 have everything that's needed to get ui tests passing on fuchsia targets.
Part of #84766. Note that this change only includes the header on tests which need an unwinder to _build_, not those which need it to _run_.
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