Move bootstrap configuration to library workspace
This creates a new "dist" profile in the standard library which contains configuration for the distributed std artifacts previously contained in bootstrap, in order for a future build-std implementation to use. bootstrap.toml settings continue to override these defaults, as would any RUSTFLAGS provided. I've left some cargo features driven by bootstrap for a future patch.
Unfortunately, profiles aren't expressive enough to express per-target overrides, so [this risc-v example](c8f22ca269/src/bootstrap/src/core/build_steps/compile.rs (L692)) was not able to be moved across. This could go in its own profile which Cargo would have to know to use, and then the panic-abort rustflags overrides would need duplicating again. Doesn't seem like a sustainable solution as a couple similar overrides would explode the number of lines here.
We could use a cargo config in the library workspace for this, but this then would have to be respected by Cargo's build-std implementation and I'm not yet sure about the tradeoffs there.
This patch also introduces a build probe to deal with the test crate's stability which is obviously not ideal, I'm open to other solutions here or can back that change out for now if anyone prefers.
cc @Mark-Simulacrum https://github.com/rust-lang/rfcs/pull/3874
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