[clang] [libcxx] [lld] [llvm] Triple::normalize: Set OS for 3-component triple with none as middle (PR #89638)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The description is a bit difficult to parse. I'll suggest that using some grammar checker to fix the description.
But thanks for putting up the patch. I think it is desired.

https://discourse.llvm.org/t/rfc-baremetal-target-triple-normalization/78524/8

> The number of none-unknown test changes actually looks manageable.
The worst case is that we notice some backward compatibility story that we don’t account for today,
and we add temporary workarounds for clang --target=aarch64-none-elf to probe lib/aarch64-none-unknown-elf (old LLVM_ENABLE_PER_TARGET_RUNTIME_DIR hierarchy)
beside lib/aarch64-unknown-none-elf.

https://github.com/llvm/llvm-project/pull/89638