Replace all __attribute__((weak)) with weak_data/weak_function

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

arch/arm/src/rtl8720c/ameba_heap.c

@@ -59,7 +59,7 @@ extern uint32_t __sram_end__;
  * address.
  */
 
-const uintptr_t __attribute__((weak)) g_idle_topstack =
+const uintptr_t weak_data g_idle_topstack =
   ((uintptr_t)&__stack);
 
 /****************************************************************************

arch/xtensa/src/esp32/esp32_spiram.c

@@ -90,7 +90,7 @@ static bool spiram_inited = false;
  * no memory is reserved if no himem function is used.
  */
 
-size_t __attribute__((weak)) esp_himem_reserved_area_size(void)
+size_t weak_function esp_himem_reserved_area_size(void)
 {
   return 0;
 }