Auto merge of #39218 - xen0n:syntax-warts, r=alexcrichton

syntax: remove abi::Os and abi::Architecture

They're long dead since the switch to flexible targets, but was not removed like their consumers were. Interesting they even got maintained by various porters out there!

Technically [syntax-breaking] as they're public API, but since they're unused in the compiler, the potential breakage IMO should be minimal.