Regression test: support big and little endian

[tautschnig]

Extracting a value from a union via a field of different size is sensitive to endianness.

• 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.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.35%. Comparing base (66ae03f) to head (ec2239c).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8370   +/-   ##
========================================
  Coverage    78.34%   78.35%           
========================================
  Files         1726     1726           
  Lines       188437   188437           
  Branches     18244    18244           
========================================
+ Hits        147638   147654   +16     
+ Misses       40799    40783   -16     

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