Add test for rust-lang#50176

Author: mati865

src/test/run-make-fulldeps/mingw-export-call-convention/Makefile

@@ -0,0 +1,9 @@
+include ../tools.mk
+
+# only-windows-gnu
+
+all:
+	$(RUSTC) foo.rs
+	# FIXME: we should make sure __stdcall calling convention is used here
+	# but that only works with LLD right now
+	nm -g "$(call IMPLIB,foo)" | $(CGREP) bar

src/test/run-make-fulldeps/mingw-export-call-convention/foo.rs

@@ -0,0 +1,4 @@
+#![crate_type = "cdylib"]
+
+#[no_mangle]
+pub extern "system" fn bar() {}

src/test/run-make-fulldeps/tools.mk

@@ -48,6 +48,7 @@ ifdef IS_MSVC
 STATICLIB = $(TMPDIR)/$(1).lib
 STATICLIB_GLOB = $(1)*.lib
 else
+IMPLIB = $(TMPDIR)/lib$(1).dll.a
 STATICLIB = $(TMPDIR)/lib$(1).a
 STATICLIB_GLOB = lib$(1)*.a
 endif