add missing files (#8124)

(cherry picked from commit 3fba8759)

testsuite/tests/rts/T8124.hs

+module T8124 where
+
+f :: Int -> Int
+f x = x + 1
+
+foreign export ccall "f" f :: Int -> Int

testsuite/tests/rts/T8124_c.c

+#include <stdlib.h>
+#include "T8124_stub.h"
+#include "HsFFI.h"
+#include <pthread.h>
+
+void *thread(void *param)
+{
+    f(3);
+    hs_thread_done();
+    pthread_exit(NULL);
+}
+
+int main (int argc, char *argv[])
+{
+    hs_init(&argc,&argv);
+
+    // check that we can call hs_thread_done() without having made any
+    // Haskell calls:
+    hs_thread_done();
+
+    // check that we can call hs_thread_done() and then make another Haskell
+    // call:
+    int i;
+    for (i=0; i < 1000; i++) {
+        f(3);
+        hs_thread_done();
+    }
+
+    // check that we can call hs_thread_done() twice:
+    hs_thread_done();
+    hs_thread_done();
+
+    // check that hs_thread_done() from child threads works:
+    pthread_t pid;
+    for (i=0; i < 1000; i++) {
+        pthread_create(&pid, NULL, thread, NULL);
+        pthread_join(pid, NULL);
+    }
+
+    hs_exit();
+    exit(0);
+}