HADOOP-12495. Fix posix_spawn error on OS X (aw)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/Shell.java

@@ -817,6 +817,9 @@ public abstract class Shell {
       return;
     }
     exitCode = 0; // reset for next run
+    if (Shell.MAC) {
+      System.setProperty("jdk.lang.Process.launchMechanism", "POSIX_SPAWN");
+    }
     runCommand();
   }