Redirect invocations of native tools

[karkhaz]

Many builds ignore $CC, $AR etc. and try to call native tools in /usr/bin. Instead of allowing the build to fail, we should log a warning and then redirect the invocation to the toolchain version of the tool.

This is so that we can get further in the build process, while gaining insight into the mistakes that builds make. For the paper, it will also allow us to present this as a prototype mitigation, and claim that x% of builds now work if we redirect the invocation.

[karkhaz]

This was implemented in #94.