[clang] [llvm] [Triple] Make a target triple "os" for firmware (PR #176272)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd approved this pull request.

I think that this is in pretty good shape now. Hopefully we didn't miss a case where this can get through on other vendors, but I think that this is careful enough to be okay.

https://github.com/llvm/llvm-project/pull/176272