Support Rust programming language Helloworld?

Duo
1

Great to have such rust app.

2

看到工具链用的是musl，但是rust里riscv64支持三种：
riscv64gc-unknown-linux-gnu
riscv64gc-unknown-none-elf
riscv64imac-unknown-none-elf

不知道哪种合适？

3

riscv64gc-unknown-linux-musl in Tier 3