Use an LLVM built sysroot instead of a clif built sysroot for user releases

[bjorn3]

This seems to improve build times on the rustc benchmark suite by a considerable amount. Likely due to having to link less code. In addition cg_clif as distributed with rustc will use an LLVM built sysroot. Local testing should still happen using a cg_clif sysroot for better test coverage. CI should test both configurations.

• Make sure is_*_feature_detected!() returns false for all target features unsupported by cg_clif. (How to handle missing SIMD support with a future rustup component #1289)
• Test both cg_clif and llvm sysroot on CI. (Implement all vendor intrinsics used by regex on AVX2 systems #1378)
• Ensure absence of miscompilations when mixing cg_clif with an LLVM built sysroot. (Miscompilation of std::thread::scope on x86_64 with llvm sysroot #1395, Tracking issue for abi-cafe failures #1525)
• Flip the switch for user releases.