libdl/alloc: Add a locking interface to the allocator.

- Allow an allocator to lock the allocations. This is needed to
  lock the heap allocator so the text and trampoline table are
  as close together as possible to allow for the largest possible
  object file size.

- Update the default heap allocator to lock the heap allocator.

- Update ELF loading to lock the allocator.

Updates #3685

cpukit/libdl/rtl-allocator.c

@@ -107,6 +107,40 @@ rtems_rtl_alloc_wr_enable (rtems_rtl_alloc_tag tag, void* address)
   rtems_rtl_unlock ();
 }
 
+void
+rtems_rtl_alloc_lock (void)
+{
+  rtems_rtl_data* rtl = rtems_rtl_lock ();
+
+  if (rtems_rtl_trace (RTEMS_RTL_TRACE_ALLOCATOR))
+    printf ("rtl: alloc: lock\n");
+
+  if (rtl != NULL)
+    rtl->allocator.allocator (RTEMS_RTL_ALLOC_LOCK,
+                              RTEMS_RTL_ALLOC_OBJECT, /* should be ignored */
+                              NULL,
+                              0);
+
+  rtems_rtl_unlock ();
+}
+
+
+void
+rtems_rtl_alloc_unlock (void)
+{
+  rtems_rtl_data* rtl = rtems_rtl_lock ();
+
+  if (rtems_rtl_trace (RTEMS_RTL_TRACE_ALLOCATOR))
+    printf ("rtl: alloc: unlock\n");
+
+  if (rtl != NULL)
+    rtl->allocator.allocator (RTEMS_RTL_ALLOC_UNLOCK,
+                              RTEMS_RTL_ALLOC_OBJECT, /* should be ignored */
+                              NULL,
+                              0);
+
+  rtems_rtl_unlock ();
+}
 void
 rtems_rtl_alloc_wr_disable (rtems_rtl_alloc_tag tag, void* address)
 {