Add LinuxBionic toolchain to Rust

The LinuxBionic toolchain wasn't defined for Rust.

This would lead to build breakage if a CC module that targeted Linux
Bionic linked against a Rust FFI module.

Bug: 174873186
Test: Build breakage no longer occurs if CC module with Rust dependency
      is built for the Linux Bionic target.
Change-Id: I39df7b9a29372986c9beeb1fe5602140d805d731