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

[YunQiang Su via cfe-commits]

wzssyqa wrote:

> I've just posted an RFC to discourse about how to handle triple normalization: https://discourse.llvm.org/t/rfc-baremetal-target-triple-normalization/78524

We don't need to revert https://github.com/llvm/llvm-project/pull/89234
It is easy to fix `Triple::normalize` once you decide how to normalize your triples.
I will help to do so.

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