Add AUR link:...

Add AUR link: [https://aur.archlinux.org/packages/riscv-gnu-toolchain/](https://aur.archlinux.org/packages/riscv-gnu-toolchain/)

Added:

Also available for Arch users on the AUR: [https://aur.archlinux.org/packages/riscv-gnu-toolchain/](https://aur.archlinux.org/packages/riscv-gnu-toolchain/)