Merge #70

70: Add docs.rs document build metadata for build targets r=Disasm a=luojia65

This pull request changes build targets when build documents on `docs.rs`, it will show RISC-V targets other than default x86 targets.

Before:

![image](https://user-images.githubusercontent.com/40385009/116803431-2add2e00-ab4a-11eb-87ea-b1cd204299d2.png)

After (take `rustsbi` crate as example): 

![image](https://user-images.githubusercontent.com/40385009/116803427-2284f300-ab4a-11eb-8974-24126cbc4310.png

)



Co-authored-by: luojia65 <me@luojia.cc>