rewrite extern-fn-mangle to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -30,7 +30,6 @@ run-make/extern-diff-internal-name/Makefile
 run-make/extern-flag-disambiguates/Makefile
 run-make/extern-flag-pathless/Makefile
 run-make/extern-fn-generic/Makefile
-run-make/extern-fn-mangle/Makefile
 run-make/extern-fn-reachable/Makefile
 run-make/extern-fn-with-union/Makefile
 run-make/extern-multiple-copies/Makefile

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

@@ -0,0 +1,16 @@
+// In this test, the functions foo() and bar() must avoid being mangled, as
+// the external C function depends on them to return the correct sum of 3 + 5 = 8.
+// This test therefore checks that the compiled and executed program respects the
+// #[no_mangle] flags successfully.
+// See https://github.com/rust-lang/rust/pull/15831
+
+//@ 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("test");
+    rustc().input("test.rs").run();
+    run("test");
+}