Rollup merge of #135340 - obeis:explicit-extern-abis, r=traviscross,nadrieril

Add `explicit_extern_abis` Feature and Enforce Explicit ABIs

The unstable `explicit_extern_abis` feature is introduced, requiring explicit ABIs in `extern` blocks. Hard errors will be enforced with this feature enabled in a future edition.

RFC rust-lang/rfcs#3722

Update #134986

Author: matthiaskrgr