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

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Oh wait, this is limited to the Darwin platform? In that case, can we have some negative tests to ensure that something like `aarch64-unknown-linux-firmware` or `aarch64-unknown-firmware` is not processed as a valid triple?

Firmware definitely sounds like it would be generally applicable to other platforms, could you include Apple/Darwin in the name of functions to make it clear that this is just for Apple platforms? As far as I can tell most other embedded operating systems just use `unknown/none-elf ` for ELF platforms and that works fine.

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