rewrite extern-fn-with-union to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -21,7 +21,6 @@ run-make/export-executable-symbols/Makefile
 run-make/extern-diff-internal-name/Makefile
 run-make/extern-flag-disambiguates/Makefile
 run-make/extern-fn-reachable/Makefile
-run-make/extern-fn-with-union/Makefile
 run-make/extern-multiple-copies/Makefile
 run-make/extern-multiple-copies2/Makefile
 run-make/fmt-write-bloat/Makefile

tests/run-make/extern-fn-generic/rmake.rs

@@ -2,6 +2,7 @@
 // test simply runs a Rust program containing generics that is also reliant on
 // a C library, and checks that compilation and execution are successful.
 // See https://github.com/rust-lang/rust/pull/15831
+
 //@ ignore-cross-compile
 // Reason: the compiled binary is executed
 

tests/run-make/extern-fn-with-union/rmake.rs

@@ -0,0 +1,16 @@
+// If an external function from foreign-function interface was called upon,
+// its attributes would only be passed to LLVM if and only if it was called in the same crate.
+// This caused passing around unions to be incorrect.
+// See https://github.com/rust-lang/rust/pull/14191
+
+//@ ignore-cross-compile
+// Reason: the compiled binary is executed
+
+use run_make_support::{build_native_static_lib, run, rustc};
+
+fn main() {
+    build_native_static_lib("ctest");
+    rustc().input("testcrate.rs").run();
+    rustc().input("test.rs").run();
+    run("test");
+}