implement primitive heap dump facility for memory profiling, accessible via Runtime.dumpHeap

The proper way to do this is to implement a subset of JVMTI, but this
will do the job for now.

classpath/java/lang/Runtime.java

@@ -75,6 +75,8 @@ public class Runtime {
 
   public native long totalMemory();
 
+  public static native void dumpHeap(String outputFile);
+
   private static class MyProcess extends Process {
     private long pid;
     private final int in;