build: add universal Mac make target

[ohill]

Summary

This adds a new universal build target that builds two libsodium libraries, runs the make build target for two architectures, and then uses lipo to merge the binaries for amd64 and arm64 together.

Test Plan

Not sure what is next, or how this would integrate into the release process. The universal binaries are roughly twice the size, but if that's a problem, the build target produces 3 bin directories (bin-darwin-amd64, bin-darwin-arm64, bin-darwin-universal) allowing for different release options.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.85%. Comparing base (f06a86e) to head (217750e).
Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6023      +/-   ##
==========================================
- Coverage   55.90%   55.85%   -0.05%     
==========================================
  Files         482      482              
  Lines       68436    68436              
==========================================
- Hits        38258    38225      -33     
- Misses      27583    27612      +29     
- Partials     2595     2599       +4     

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[onetechnical]

Thanks for this! It'd be nice to provide a single binary for Mac OS, and will help with AMD64 deprecations.