Better support for non-root runs

[travisdowns]

You can currently run uarch-bench as non-root, but the experience isn't great (for example, you have to run the uarch-bench binary directly rather than use the wrapper script). We should make this better, e.g., allowing users to still use the wrapper script with NOROOT=1 or something like that.

In that case we don't try to do things that require root and warn where this may affect the stability of results.