Added Serial4 (USART2) and updated compiler

[vChavezB]

Dear arduino team,

I have been working with the Arduino Due for some research projects and due to the lack of a Serial4 support and the latest compiler (10.3-2021-10) I have modified the sdk. In addition I added a callback for the USART/UART RX irq since I required to process information in real-time.

I kindly ask you to review the changes to check what could be deemed acceptable to add and what not. I know that updating the compiler might break some projects or bring strange behaviors due to unknown compiler bugs from the older versions. This is a risk but I think that as Arduino is a prototyping environment it should use the latest tools.

Regards,
Victor

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ vChavezB
❌ Victor

---

Victor seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[vChavezB]

Something tells me the Arduino Due is no longer mantained, so I will close this PR.