Switch the OSX universal binary build to X86 + ARM64

[kroening]

This adds support for ARM64 and removes the support for i386. Support for
i386 has been removed from XCode, and the building with this target fails
owing to missing libraries for this architecture.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5736 (ec939e1) into develop (ba467ef) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5736   +/-   ##
========================================
  Coverage    69.49%   69.49%           
========================================
  Files         1243     1243           
  Lines       100701   100701           
========================================
  Hits         69985    69985           
  Misses       30716    30716           

Flag	Coverage Δ
cproversmt2	43.26% <ø> (ø)
regression	66.39% <ø> (ø)
unit	32.25% <ø> (ø)

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