mimxrt/boards: Set __heap_size__ to 0 in MIMXRT1011.ld.

Do not use the traditional C heap in order to save memory, because the
traditional C heap is unused in MicroPython.

ports/mimxrt/boards/MIMXRT1011.ld

@@ -3,6 +3,9 @@ __stack_size__ = 0x6000;
 _estack = __StackTop;
 _sstack = __StackLimit;
 
+/* Do not use the traditional C heap. */
+__heap_size__ = 0;
+
 /* Use second OCRAM bank for GC heap. */
 _gc_heap_start = ORIGIN(m_data2);
 _gc_heap_end = ORIGIN(m_data2) + LENGTH(m_data2);