In rust-lang#89522 we learned that `clone3` is interacting poorly with Gentoo's
`sandbox` tool. We only need that for the unstable pidfd extensions, so
otherwise avoid that and use a normal `fork`.

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

This reverts commit 12fbabd.

It was only needed because of using raw `clone3` instead of `fork`, but
we only do that now when a pidfd is requested.

…/LangRef.html#supported-constraint-code-list

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

This is the last step in landing rust-lang/miri#1340!

Only use `clone3` when needed for pidfd

In rust-lang#89522 we learned that `clone3` is interacting poorly with Gentoo's
`sandbox` tool. We only need that for the unstable pidfd extensions, so
otherwise avoid that and use a normal `fork`.

This is a re-application of beta rust-lang#89924, now that we're aware that we need
more than just a temporary release fix. I also reverted 12fbabd, as
that was just fallout from using `clone3` instead of `fork`.

r? `@Mark-Simulacrum`
cc `@joshtriplett`

…=Amanieu

adjust documented inline-asm register constraints

This change more clearly specifies how `reg` and `reg_thumb` work with ARM, Thumb2, and Thumb1 code.

Based upon the [llvm documentation](https://llvm.org/docs/LangRef.html#supported-constraint-code-list) for register constraint codes.
To be clear, this just updates the docs to match what already happens with rustc/llvm.
No change in the compiler is required to make it match this new documentation.

Update Miri

Fixes rust-lang#90763.

This is the last step in landing rust-lang/miri#1340!

r? `@RalfJung`