Fix #5218, calculate age of Julia checkout in banner() function

After static compillation of base Julia was implemented, the startup
banner showed the age of the newest commit in (origin/)master at the
time of compillation, instead of at startup time. This commit moves the
age calculation into the Base.banner() function, and moves the function
from base/client.jl to base/version.jl in order to have access to the
right variables. (Thanks @ivarne!)