Remove dependency on CMake and Clang for riscv64

[Xeonacid]

Problem:

Thanks for the awesome project!

Please consider pregenerating aws-lc-sys bindings for riscv64 like what have done for x86_64 and aarch64.

The reason behind this is to make downstream RISC-V distros' life easier. For example, Arch Linux x86_64 doesn't need the cmake and clang dependencies while Arch Linux RISC-V needs. So we Arch Linux RISC-V has to patch PKGBUILDs to add the dependencies for every package using aws-lc-sys.

Follow-up of #714

Relevant details

System information: for linux, below info cae collected by running uname -srvmp

• CPU architecture: riscv64
• OS: Linux
• Rust triplet: riscv64gc-unknown-linux-gnu