Improve the missing_abi lint.

compiler/rustc_lint/messages.ftl

@@ -268,7 +268,7 @@ lint_extern_crate_not_idiomatic = `extern crate` is not idiomatic in the new edi
 
 lint_extern_without_abi = extern declarations without an explicit ABI are deprecated
     .label = ABI should be specified here
-    .help = the default ABI is {$default_abi}
+    .suggestion = explicitly specify the {$default_abi} ABI
 
 lint_for_loops_over_fallibles =
     for loop over {$article} `{$ref_prefix}{$ty}`. This is more readably written as an `if let` statement

compiler/rustc_lint/src/lints.rs

@@ -2738,11 +2738,9 @@ pub(crate) struct PatternsInFnsWithoutBodySub {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_extern_without_abi)]
-#[help]
 pub(crate) struct MissingAbi {
-    #[label]
+    #[suggestion(code = "extern \"{default_abi}\"", applicability = "machine-applicable")]
     pub span: Span,
-
     pub default_abi: &'static str,
 }