Detect cross compilation and adjust Cargo output directory

[alerque]

Cross compiling with Cargo adds a segment to the output directory with the host info. Currently we don't have a way to detect this and adjust, we just blindly assume the Cargo directory will be "target/release" instead of perhaps "target/x86_64-unknown-linux-gnu/release".

This is a problem for some distro packages, e.g. Void bumped into this.

Somehow we should detect whether cross compilation is being used and what the target triple is.