introduce rounding_mode_identifier()

[kroening]

This commit consolidates the various places that contain the string
CPROVER_PREFIX "rounding_mode".

• 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.

[tautschnig]

Perhaps the commit message should be a bit more accurate in that this commit does not capture the uses in the C library.

[codecov]

Codecov Report

Merging #6289 (a0a1329) into develop (63203b1) will increase coverage by 0.00%.
The diff coverage is 70.83%.

@@           Coverage Diff            @@
##           develop    #6289   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1510     1510           
  Lines       163465   163467    +2     
========================================
+ Hits        124208   124213    +5     
+ Misses       39257    39254    -3     

Impacted Files	Coverage Δ
src/jsil/jsil_entry_point.cpp	0.00% <0.00%> (ø)
src/jsil/jsil_internal_additions.cpp	0.00% <0.00%> (ø)
src/util/floatbv_expr.cpp	0.00% <0.00%> (ø)
src/util/floatbv_expr.h	73.58% <ø> (ø)
src/util/ieee_float.cpp	88.01% <0.00%> (ø)
src/util/ieee_float.h	85.71% <ø> (ø)
...java_bytecode/java_bytecode_internal_additions.cpp	100.00% <100.00%> (ø)
jbmc/src/java_bytecode/java_entry_point.cpp	89.06% <100.00%> (ø)
src/analyses/constant_propagator.cpp	95.06% <100.00%> (ø)
...yses/variable-sensitivity/abstract_environment.cpp	95.02% <100.00%> (ø)
... and 11 more

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