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-alloc-heap.c

@@ -17,17 +17,30 @@
 
 #include "rtl-alloc-heap.h"
 
+#include <rtems/score/apimutex.h>
+
 void
 rtems_rtl_alloc_heap (rtems_rtl_alloc_cmd cmd,
                       rtems_rtl_alloc_tag tag,
                       void**              address,
                       size_t              size)
 {
-  if (cmd == RTEMS_RTL_ALLOC_NEW)
-    *address = malloc (size);
-  else if (cmd == RTEMS_RTL_ALLOC_DEL)
+  switch (cmd)
   {
-    free (*address);
-    *address = NULL;
+    case RTEMS_RTL_ALLOC_NEW:
+      *address = malloc (size);
+      break;
+    case RTEMS_RTL_ALLOC_DEL:
+      free (*address);
+      *address = NULL;
+      break;
+    case RTEMS_RTL_ALLOC_LOCK:
+      _RTEMS_Lock_allocator();
+      break;
+    case RTEMS_RTL_ALLOC_UNLOCK:
+      _RTEMS_Unlock_allocator();
+      break;
+    default:
+      break;
   }
 }