Most Rust freestanding/bare-metal targets use just `-unknown-none` here,
including aarch64-unknown-none, mipsel-unknown-none, and the BPF
targets. The *only* target using `-unknown-none-elf` is RISC-V.

The underlying toolchain doesn't care; LLVM accepts both `x86_64-unknown-none`
and `x86_64-unknown-none-elf`.

In addition, there's a long history of embedded x86 targets with varying
definitions for the `elf` suffix; on some of those embedded targets,
`elf` implied the inclusion of a C library based on newlib or similar.
Using `x86_64-unknown-none` avoids any potential ambiguity there.

(Work on this target sponsored by Profian.)

(Work on this target sponsored by Profian.)

(Work on this target sponsored by Profian.)

Based on the list used for x86_64-unknown-none-linuxkernel.

(Work on this target sponsored by Profian.)

Specify the `cpu` and the `max_atomic_width` (64).

Set `stack_probes` similarly to other targets to work around known
issues, and copy the corresponding comment from those targets.

Build position-independent code that doesn't require relocations.

(Work on this target sponsored by Profian.)

In particular, document the default properties and assumptions of code
built for the target.

(Work on this target sponsored by Profian.)