Follow-up to 090790a467: attribute __used__ requires unique names

[tautschnig]

With 090790a we made sure that symbols marked with
attribute((used)) are not discarded. Linking multiple files that
use symbols marked as such still must not result in conflicting
declarations on such symbols, thus use the DJB name mangler to generate
unique names.

• 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

Base: 78.34% // Head: 78.26% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (9fd1769) compared to base (a53fa0f).
Patch coverage: 90.00% of modified lines in pull request are covered.

❗ Current head 9fd1769 differs from pull request most recent head 9298526. Consider uploading reports for the commit 9298526 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5729      +/-   ##
===========================================
- Coverage    78.34%   78.26%   -0.09%     
===========================================
  Files         1644     1642       -2     
  Lines       190313   189748     -565     
===========================================
- Hits        149097   148498     -599     
- Misses       41216    41250      +34     

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_declaration.h	95.00% <ø> (-0.24%)	⬇️
src/ansi-c/c_typecheck_base.cpp	83.78% <85.71%> (+0.19%)	⬆️
src/ansi-c/ansi_c_declaration.cpp	74.03% <100.00%> (-0.04%)	⬇️
src/goto-symex/solver_hardness.cpp	53.00% <0.00%> (-6.00%)	⬇️
src/util/byte_operators.cpp	90.00% <0.00%> (-3.11%)	⬇️
src/solvers/smt2/smt2_conv.cpp	66.43% <0.00%> (-2.18%)	⬇️
src/util/expr_initializer.cpp	83.33% <0.00%> (-1.78%)	⬇️
src/goto-programs/goto_program.cpp	80.24% <0.00%> (-1.56%)	⬇️
...ncremental/smt2_incremental_decision_procedure.cpp	98.09% <0.00%> (-0.67%)	⬇️
src/solvers/flattening/arrays.cpp	79.83% <0.00%> (-0.54%)	⬇️
... and 59 more

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