Set the nfi posix backend as default in the launcher

Author: otethal

graalpython/com.oracle.graal.python.shell/src/com/oracle/graal/python/shell/GraalPythonMain.java

@@ -563,6 +563,13 @@ protected void launch(Builder contextBuilder) {
 
         contextBuilder.option("python.CheckHashPycsMode", checkHashPycsMode);
 
+        if (getContextOptionIfSetViaCommandLine("python.PosixModuleBackend") == null && getContextOptionIfSetViaCommandLine("PosixModuleBackend") == null) {
+            // TODO find a proper way to force emulated backend in llvm.managed mode
+            if (getClass() == GraalPythonMain.class) {
+                contextBuilder.option("python.PosixModuleBackend", "native");
+            }
+        }
+
         if (multiContext) {
             contextBuilder.engine(Engine.newBuilder().allowExperimentalOptions(true).options(enginePolyglotOptions).build());
         }