Make USART IRQ handler WEAK

[NitrofMtl]

Description: This patch modifies the predefined USART IRQ handlers in the Arduino Due core to be declared attribute((weak)).

Motivation:

• Allows libraries, such as 'DUERS485DMA', to override the default USART interrupt handlers.

• Necessary for DMA-based RS485 implementations that require custom IRQ handling without modifying the core itself.

Changes:

• All predefined USARTx_Handler() functions are now weak.

• Existing functionality remains unchanged if no library overrides the handler.

• No effect on boards or configurations that do not use these handlers.

Impact:

• Compatible with existing sketches using standard Serial objects.

• Enables advanced libraries to implement DMA or custom serial processing.

Notes:

• Only the weak attribute is added; the logic inside the handlers is untouched.

• Users may override handlers selectively (only the USARTs they use).