Move rounding_mode_identifier() to configt

[tautschnig]

This removes a goto-programs dependency from linking/, which is all
about symbol tables, not goto programs.

Depends on #6554 for all CI checks to pass.

• 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 80.39%. Comparing base (22d25d1) to head (96dfda9).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6555      +/-   ##
===========================================
- Coverage    80.39%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207403   207404       +1     
  Branches        73       73              
===========================================
- Hits        166752   166750       -2     
- Misses       40651    40654       +3     

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