gitserver: build p4-fusion as a seperate stage

Created by: keegancsmith

When we introduced p4-fusion to our docker images it added a significant amount of build time. In the case of server it doubled the build time. This is a first step to improving that but only targetted at gitserver. Next up we can do the same change to server, but I wanted to keep this focussed.

The meat of this change is extracting the docker steps for p4-fusion into a build script. This is in preparation for using it in server image builds.

The main benefit is it is now a seperate change. Previously we ran into linking issues but this was easy to resolve. You just need to install libstdc++.

Note: I noticed some flaws in our build script. This commit doesn't fix those it just does manual refactoring. Follow-up commits can improve the process. The only real functional change it does is use a shell script trap for cleanup.

Test Plan: A lot of docker building. Running the gitserver image and checking that p4-fusion runs and "ldd p4-fusion" is happy.