protect ieee_floatt::rounding_mode

[kroening]

To ensure complete initialization, the rounding_mode member of ieee_floatt is now protected.

• 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/
• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.93%. Comparing base (f9a7807) to head (1a1b6ac).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8551      +/-   ##
===========================================
+ Coverage    78.38%   78.93%   +0.55%     
===========================================
  Files         1729     1729              
  Lines       200151   198762    -1389     
  Branches     18244    18313      +69     
===========================================
+ Hits        156883   156888       +5     
+ Misses       43268    41874    -1394     

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