use existing overloadable Pico SDK section macro instead of creating a custom one

Author: majbthrd

src/common/tusb_mcu.h

@@ -442,7 +442,7 @@
   #define TUP_DCD_ENDPOINT_MAX    16
   #define TUP_MCU_MULTIPLE_CORE   1
 
-  #define TU_ATTR_FAST_FUNC       __attribute__((section(".time_critical.tinyusb")))
+  #define TU_ATTR_FAST_FUNC       __not_in_flash("tinyusb")
 
 //--------------------------------------------------------------------+
 // Silabs