Add riscv64 Support

[liberodark]

Hi,

Please can you add riscv64 support on your release ?
Also have riscv64 server if you want.
Here : https://open-riscv-initiative.com/

Best Regards

[XuShaohua]

Today I've spent several hours to migrate crates which rustdesk depends on to riscv64 architecture.
And now cargo build successfully.

But neither sciter-sdk nor flutter supports riscv64 currently.
It's a pity

By the way, compiling packages on riscv64 is too slow.