use a proper shared library for the core compiler intead of a ji file

[KristofferC]

This should be faster (ref JuliaLang/julia@095fc99)

Just a random example (with the second step being the important one):

Before:

julia> PackageCompiler.create_sysimage(["Glob"]; sysimage_path="glob.dylib", incremental=false)
✔ [02m:27s] PackageCompiler: creating compiler .ji image (incremental=false)
✔ [01m:31s] PackageCompiler: compiling fresh sysimage (incremental=false)
✔ [00m:29s] PackageCompiler: compiling nonincremental system image

After:

julia> PackageCompiler.create_sysimage(["Glob"]; sysimage_path="glob.dylib", incremental=false)
✔ [02m:35s] PackageCompiler: creating compiler sysimage (incremental=false)
✔ [01m:05s] PackageCompiler: compiling fresh sysimage (incremental=false)
✔ [00m:28s] PackageCompiler: compiling nonincremental system image

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.21%. Comparing base (cfb6002) to head (0cfd466).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1060      +/-   ##
==========================================
+ Coverage   86.16%   86.21%   +0.05%     
==========================================
  Files           3        3              
  Lines         795      798       +3     
==========================================
+ Hits          685      688       +3     
  Misses        110      110              

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[topolarity]

Big improvement on Julia nightly for me:

✔ [04m:46s] PackageCompiler: creating compiler sysimage (incremental=false)
✔ [03m:19s] PackageCompiler: compiling fresh sysimage (incremental=false)
✔ [02m:32s] PackageCompiler: compiling nonincremental system image

vs. without PR:

✔ [04m:23s] PackageCompiler: creating compiler .ji image (incremental=false)
✔ [19m:55s] PackageCompiler: compiling fresh sysimage (incremental=false)
✔ [02m:36s] PackageCompiler: compiling nonincremental system image

In the last compiling incremental system image step, there's still another big slowdown (vs. 1.12) for this workload. I'll try to bisect that soon.