Flag/Env var to set --cpu-target #804

Is your feature request related to a problem? Please describe.
I am trying to maximise the use of the compile cache on github actions.
A thing I learnt to day about github actions, is that it can run on multiple different types of machines with multiple different micro-archetectures, and that this breaks the compile cache.
But i believe if julia is always run with julia --cpu-target x86-64-v3 that causes compilation to always build for something like the minimal feature set common to all modern machines. (can go down to older versions too).
This should make precompilation caches more portable.

Describe the solution you'd like
I think what has to be one is add
CONFIG["opt_cpu_target"] = option('cpu_target')[0]
into the source code here
I think the rest of the parse-opt machinery would handle it.