Use aarch64-apple-darwin as the fallback doc source for -apple-

We are moving away from `x86_64-apple-darwin`, so soon these docs
won't be available.

(cherry picked from commit 36a38206db)

src/tools/build-manifest/src/main.rs

@@ -203,7 +203,7 @@ static TARGETS: &[&str] = &[
 ///
 /// The order here matters, more specific entries should be first.
 static DOCS_FALLBACK: &[(&str, &str)] = &[
-    ("-apple-", "x86_64-apple-darwin"),
+    ("-apple-", "aarch64-apple-darwin"),
     ("aarch64", "aarch64-unknown-linux-gnu"),
     ("arm-", "aarch64-unknown-linux-gnu"),
     ("", "x86_64-unknown-linux-gnu"),