implement Runtime.maxMemory for OpenJDK port

Some apps refuse to run if Runtime.maxMemory returns a value that's
"too small", so our stub implementation returning zero was not
sufficient.  Now we return the actual heap size limit in bytes.

src/classpath-openjdk.cpp

@@ -3116,7 +3116,7 @@ EXPORT(JVM_FreeMemory)()
 extern "C" JNIEXPORT jlong JNICALL
 EXPORT(JVM_MaxMemory)()
 {
-  return 0;
+  return local::globalMachine->heap->limit();
 }
 
 extern "C" JNIEXPORT jint JNICALL

src/heap.cpp

@@ -1867,6 +1867,10 @@ class MyHeap: public Heap {
     c.immortalHeapEnd = start + sizeInWords;
   }
 
+  virtual unsigned limit() {
+    return c.limit;
+  }
+
   virtual bool limitExceeded() {
     return c.count > c.limit;
   }

src/heap.h

@@ -59,6 +59,7 @@ class Heap: public Allocator {
 
   virtual void setClient(Client* client) = 0;
   virtual void setImmortalHeap(uintptr_t* start, unsigned sizeInWords) = 0;
+  virtual unsigned limit() = 0;
   virtual bool limitExceeded() = 0;
   virtual void collect(CollectionType type, unsigned footprint) = 0;
   virtual void* allocateFixed(Allocator* allocator, unsigned sizeInWords,