Allow user to override default value of cpu-target

[DilumAluthge]

I build Julia from source on macOS. My default system image does not have the generic cpu-target available. However, it does have the native cpu-target available. So I would like to be able to specify native as the cpu-target to use.

This pull request allows you to pass the cpu-target as the keyword argument cpu_target.

cc: @KristofferC

[KristofferC]

This has some overlap with #43

[KristofferC]

In short, sysimages should have native default target while apps should have the same one as we bundle julia with ("generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)").

[DilumAluthge]

Yeah it seems that #43 is this PR plus some changing of the default targets.

The "change the default targets" part of #43 seems to break CI because of the issue relating the @ccallable that you opened on the Julia repo.

What do you think about merging this now, and then finishing up #43 once JuliaLang/julia#34076 et al are fixed?

[KristofferC]

The error in that PR is due to an unrelated change (adding some extra test coverage) so I will take that out and merge it.

[codecov-io]

Codecov Report

Merging #52 into master will decrease coverage by 0.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   90.69%   90.33%   -0.36%     
==========================================
  Files           2        2              
  Lines         215      207       -8     
==========================================
- Hits          195      187       -8     
  Misses         20       20

Impacted Files	Coverage Δ
src/PackageCompilerX.jl	90.21% <100%> (-0.36%)	⬇️
src/juliaconfig.jl	91.3% <0%> (-0.37%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 447960d...d42cffb. Read the comment docs.

[DilumAluthge]

Superseded by #43