cpukit/libcsupport: Implement malloc_usable_size

Implement malloc_usable_size to get the underlying heap block size of a
memory address on the heap.

Closes #4503

cpukit/libcsupport/src/malloc.c

@@ -40,6 +40,7 @@
 #ifdef RTEMS_NEWLIB
 #include <stdlib.h>
 #include <errno.h>
+#include <malloc.h>
 
 #include "malloc_p.h"
 
@@ -62,4 +63,38 @@ void *malloc(
   return return_this;
 }
 
+size_t malloc_usable_size(
+  void *area
+)
+{
+  Heap_Block* block = NULL;
+  size_t size = 0;
+  bool needs_lock;
+
+  if ( area == NULL ) {
+    return 0;
+  }
+
+  needs_lock = _Malloc_System_state() == MALLOC_SYSTEM_STATE_NORMAL;
+
+  if ( needs_lock ) {
+    _RTEMS_Lock_allocator();
+  }
+
+  block = _Heap_Block_of_alloc_area(
+    (uintptr_t)area,
+    RTEMS_Malloc_Heap->page_size
+  );
+
+  if (block) {
+    size = _Heap_Block_size(block);
+  }
+
+  if ( needs_lock ) {
+    _RTEMS_Unlock_allocator();
+  }
+
+  return size;
+}
+
 #endif