need precompile statements re-enabled for `addprocs` (with PR)

As discovered in https://discourse.julialang.org/t/help-with-binary-trees-benchmark-games-example/37307/13

❯ hyperfine -w1 "julia -p4 -E 'using Distributed; nprocs()'" "julia -E 'using Distributed; addprocs(); nprocs()'"
Benchmark JuliaLang/julia#1: julia -p4 -E 'using Distributed; nprocs()'
  Time (mean ± σ):      2.040 s ±  0.010 s    [User: 5.563 s, System: 0.773 s]
  Range (min … max):    2.024 s …  2.054 s    10 runs
 
Benchmark JuliaLang/julia#2: julia -E 'using Distributed; addprocs(); nprocs()'
  Time (mean ± σ):      1.785 s ±  0.014 s    [User: 5.337 s, System: 0.756 s]
  Range (min … max):    1.765 s …  1.816 s    10 runs
 
Summary
  'julia -E 'using Distributed; addprocs(); nprocs()'' ran
    1.14 ± 0.01 times faster than 'julia -p4 -E 'using Distributed; nprocs()''

Is there a reason spawning the extra processes with addprocs() is necessarily faster than spawning them with -p command-line argument?

Start by reproducing the supplied hyperfine comparison between julia -p4 and addprocs() with Distributed. Then trace how the two startup paths handle precompilation and determine whether the precompile statements should be re-enabled for addprocs; done means the behavior is explained and the relevant startup performance is verified.