allow building a "fat sysimage" with all the stdlibs in it

[KristofferC]

This is an alternative to #60139 where instead of trying to put the stdlibs into the first sysimage (which requires running their top-level code in the output-o process which doesn't run inits), we do like PackageCompiler and build a new sysimage on top of the "base" one where we can load already precompiled stdlibs. This allows us to put e.g. REPL into the sysimage which does not work the "normal" method.

❯ ./julia -e '@time using REPL; @time using Pkg'                                 
  0.002100 seconds (499 allocations: 35.359 KiB)
  0.000432 seconds (373 allocations: 30.516 KiB)

The makefile stuff is completely AI generated.

cc @pankgeorg