Allow libwebrtc to build with host toolchain on Linux

[hack3ric]

Introduces a new parameter --toolchain whose value defaults to sysroot, but can be changed to host to build libwebrtc in the host environment.

This could be later used in webrtc-sys' build.rs so that WebRTC could be built natively and conveniently inside cargo build, when building everything from source is desired (e.g. in a Linux distro).

[CLAassistant]

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[Be-ing]

Thank you for starting this. It no longer works when rebased on main since libwebrtc was updated in #696. I have made another PR which works on the current main branch: #730.