Add support for aarch64-apple-darwin

[odow]

Closes #193

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.49%. Comparing base (f350266) to head (f7e31ac).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Cbc.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
+ Coverage   94.15%   94.49%   +0.33%     
==========================================
  Files           2        2              
  Lines         359      363       +4     
==========================================
+ Hits          338      343       +5     
+ Misses         21       20       -1     

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