Add an `abi` field to `TargetOptions`, defaulting to "". Support using
`cfg(target_abi = "...")` for conditional compilation on that field.
Gated by `feature(cfg_target_abi)`.
Add a test for `target_abi`, and a test for the feature gate.
Add `target_abi` to tidy as a platform-specific cfg.
This does not add an abi to any existing target.
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_.
r? ````@Mark-Simulacrum````
