Information about host machine architecture leaks into sandbox

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/262)
In GitLab by [[Gitlab user @samthursfield]](https://gitlab.com/samthursfield) on Feb 20, 2018, 11:50

On the Linux platform at least, processes within the sandbox have access to the host's CPU info (`/proc/cpuinfo` and on x86 the `cpuid` instruction).

If build systems in the sandbox use `gcc -march=native` or something similar, reproducibility will be compromised and the built artifacts might not work on CPUs with a different feature set.

Start by investigating how the Linux sandbox exposes /proc/cpuinfo and the x86 cpuid instruction. Confirm the current visibility from processes inside the sandbox and determine the project’s existing reproducibility expectations; done means host CPU features are no longer available to sandboxed builds.

Written by the indexing model from the issue text.