Use endianness map for both big and little endian

[tautschnig]

Avoid special-casing little endianness, and use endianness map for union
member access.

• Each commit message has a non-empty body, explaining why the change was made.
• 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

Merging #5859 (fba1372) into develop (36151cf) will decrease coverage by 1.34%.
The diff coverage is 54.90%.

❗ Current head fba1372 differs from pull request most recent head e7bae74. Consider uploading reports for the commit e7bae74 to get more accurate results

@@             Coverage Diff             @@
##           develop    #5859      +/-   ##
===========================================
- Coverage    74.23%   72.89%   -1.35%     
===========================================
  Files         1444     1423      -21     
  Lines       157309   154242    -3067     
===========================================
- Hits        116781   112432    -4349     
- Misses       40528    41810    +1282     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv.h	69.56% <ø> (ø)
src/solvers/flattening/boolbv_with.cpp	68.08% <0.00%> (+2.77%)	⬆️
src/solvers/flattening/boolbv_member.cpp	53.65% <35.71%> (-41.08%)	⬇️
src/solvers/flattening/boolbv_union.cpp	86.66% <83.33%> (+29.52%)	⬆️
src/solvers/flattening/boolbv.cpp	76.67% <100.00%> (-2.61%)	⬇️
src/solvers/flattening/boolbv_byte_update.cpp	100.00% <100.00%> (ø)
src/util/find_symbols.h	0.00% <0.00%> (-100.00%)	⬇️
src/goto-instrument/wmm/data_dp.h	0.00% <0.00%> (-100.00%)	⬇️
src/goto-instrument/accelerate/path.h	0.00% <0.00%> (-100.00%)	⬇️
src/goto-instrument/accelerate/accelerate.h	0.00% <0.00%> (-100.00%)	⬇️
... and 338 more

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