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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

I have tested building a toolchain using `aarch64-none-elf`, and the problems described in my comment on https://github.com/llvm/llvm-project/pull/89234#issuecomment-2069019606 are resolved. The only change I had to make was to update some references to `aarch64-none-elf` to reference `aarch64-unknown-none-elf`, reflecting the change in compiler-rt install paths introduced in #89234.

I concur with Sander that it seems prudent to have the functional change in its own commit, in case it requires further revert flip-flop.

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