Rollup merge of #126481 - ChrisDenton:powerpc-unkown-openbsd, r=ehuss

Add `powerpc-unknown-openbsd` maintenance status

As noted in rust-lang/rust#126451 (comment) `powerpc-unknown-openbsd` is not maintained by the OpenBSD devs. If a maintainer is found this can be updated with their information but we should document the current status and note explicitly that it's different from other OpenBSD targets.