Add `linux-riscv64` support

[alexrp]

Blocked on:

• Add a linux-riscv64 toolset zig-toolsets#22

[alexrp]

The toolset package is now published.

[alexrp]

This won't be worth pursuing for a while, as there is still significant upstream work needed: ziglang/zig#4456

That said, progress on the .NET side is looking very promising. 👀