in _gettid use the one ctx.mainthread id for all ctx threads

Author: tomasstupka

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/MultiprocessingModuleBuiltins.java

@@ -210,8 +210,8 @@ long spawn(VirtualFrame frame, int fd, int sentinel, PList keepFds,
     abstract static class GetTidNode extends PythonBuiltinNode {
         @Specialization
         @TruffleBoundary
-        static long getTid() {
-            return convertTid(Thread.currentThread().getId());
+        long getTid() {
+            return convertTid(getContext().getMainThread().getId());
         }
     }