Add explicit_extern_abis unstable feature

Author: obeis

compiler/rustc_feature/src/unstable.rs

@@ -479,6 +479,8 @@ declare_features! (
     (incomplete, dyn_star, "1.65.0", Some(102425)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (unstable, exhaustive_patterns, "1.13.0", Some(51085)),
+    /// Allows explicit extern ABIs.
+    (unstable, explicit_extern_abis, "1.84.2", Some(134986)),
     /// Allows explicit tail calls via `become` expression.
     (incomplete, explicit_tail_calls, "1.72.0", Some(112788)),
     /// Allows defining `extern type`s.

compiler/rustc_span/src/symbol.rs

@@ -853,6 +853,7 @@ symbols! {
         expf16,
         expf32,
         expf64,
+        explicit_extern_abis,
         explicit_generic_args_with_impl_trait,
         explicit_tail_calls,
         export_name,