Support toolchain cleanup

[samrocketman]

Some post build actions need to occur during the post build clean up. For example, in ruby rvm should build inside of a randomly numbered gemset and when the build is finished there should be a step to clean up that gemset. This is desired when builds happen on long running slaves. Some thought should be put into how to handle this. This is less of an issue on an ephemeral build slave.

[samrocketman]

Toolchains cleanup should use a trap function which is a built-in for bash. It should be defined at the end of all toolchains. The reason being is some toolchains (I'm looking at you RVM) clear all trap functions when they're loaded.