fix issue 54153 by ignoring issue-18804 test on windows/mac.

As a drive-by, add `-C no-prepopulate-passes` as suggested by nikic.

src/test/run-pass/issues/issue-18804/main.rs

@@ -14,9 +14,14 @@
 
 // ignore-asmjs no weak symbol support
 // ignore-emscripten no weak symbol support
+// ignore-windows no extern_weak linkage
+// ignore-macos no extern_weak linkage
 
 // aux-build:lib.rs
 
+// rust-lang/rust#56772: nikic says we need this to be proper test.
+// compile-flags: -C no-prepopulate-passes
+
 extern crate lib;
 
 fn main() {