patch 9.0.0589: on AmigaOS4 the pid is available but the task address is used

Problem:    On AmigaOS4 the pid is available but the task address is used.
Solution:   Use getpid().  (Ola Söder, closes #11224)
diff --git a/src/os_amiga.c b/src/os_amiga.c
index 0e5c714..25e8956 100644
--- a/src/os_amiga.c
+++ b/src/os_amiga.c
@@ -704,7 +704,9 @@
     long
 mch_get_pid(void)
 {
-#if defined(__amigaos4__) || defined(__AROS__) || defined(__MORPHOS__)
+#if defined(__amigaos4__)
+    return (long) getpid();
+#elif defined(__AROS__) || defined(__MORPHOS__)
     // This is as close to a pid as we can come. We could use CLI numbers also,
     // but then we would have two different types of process identifiers.
     return((long)FindTask(0));