Add libwebrtc build support for riscv64

[nponsard]

This PR adds the required changes to be able to build the webrtc-sys crate on the riscv64gc-unknown-linux-gnu Rust target.

The sysroot install script of the chromium project doesn't have yet support for riscv64 so I had to install it another way, inspired from this blogpost : https://www.kxxt.dev/blog/cross-compile-chromium-for-riscv

[CLAassistant]

All committers have signed the CLA.