Bad link of riscv-rust in 0003-ckb-vm.md

[u2]

Bad link of riscv-rust in https://github.com/nervosnetwork/rfcs/blob/master/rfcs/0003-ckb-vm/0003-ckb-vm.md#overview

[doitian]

@xxuejie is https://github.com/riscv-rust/riscv-rust-toolchain the correct one?

[xxuejie]

I think we should use this link: rust-embedded/wg#218 since we are moving to 64 bit now, and Rust's RISC-V 64 bit support is still in progress.

[u2]

riscv-rust/riscv-rust-toolchain#29