ignore prim-methods-external-core rustdoc test on macOS

The test is for an unstable feature that doesn't affect the stable
release (no_core), and it's causing CI issues for macOS.

Author: pietroalbini

src/test/rustdoc/intra-doc/prim-methods-external-core.rs

@@ -2,6 +2,7 @@
 // build-aux-docs
 // ignore-cross-compile
 // ignore-windows
+// ignore-macos
 
 #![deny(broken_intra_doc_links)]
 #![feature(no_core, lang_items)]