Add linux/riscv64 arch support

[bitsk]

Hello, I am working on adding support for the RISC-V architecture to the Arduino IDE, and most of the packages depend on this environment to build. Could you consider adding RISC-V architecture cross-compilation environment support to this repository? Thank you very much.

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[mergify]

This pull request does not have a backport label. Could you fix it @bitsk? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d is the label to automatically backport to the 1./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b main upstream/main
git merge upstream/main
git push upstream main