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.)
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