rewrite extern-fn-generic to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -20,7 +20,6 @@ run-make/emit-to-stdout/Makefile
 run-make/export-executable-symbols/Makefile
 run-make/extern-diff-internal-name/Makefile
 run-make/extern-flag-disambiguates/Makefile
-run-make/extern-fn-generic/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-generic/Makefile

@@ -1,7 +0,0 @@
-# ignore-cross-compile
-include ../tools.mk
-
-all: $(call NATIVE_STATICLIB,test)
-	$(RUSTC) testcrate.rs
-	$(RUSTC) test.rs
-	$(call RUN,test) || exit 1

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

@@ -0,0 +1,15 @@
+// Generic types in foreign-function interfaces were introduced in #15831 - this
+// 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
+
+use run_make_support::{build_native_static_lib, run, rustc};
+
+fn main() {
+    build_native_static_lib("test");
+    rustc().input("testcrate.rs").run();
+    rustc().input("test.rs").run();
+    run("test");
+}