C library: whitespace fix to make fmaxf, fmaxl available

[tautschnig]

We parse the comments for "FUNCTION:" and therefore must not have a
space before the colon.

Enable and add regression tests for all variants of fmax (and also its
dual fmin, while at it).

• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6904 (840c4ec) into develop (5ea97b3) will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6904      +/-   ##
===========================================
+ Coverage    77.77%   77.82%   +0.05%     
===========================================
  Files         1568     1568              
  Lines       180315   180004     -311     
===========================================
- Hits        140234   140092     -142     
+ Misses       40081    39912     -169     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_get.cpp	82.03% <0.00%> (-1.46%)	⬇️
src/solvers/flattening/boolbv.cpp	86.36% <0.00%> (-1.03%)	⬇️
src/pointer-analysis/value_set_dereference.cpp	96.40% <0.00%> (-0.33%)	⬇️
src/util/simplify_expr_class.h	90.47% <0.00%> (ø)
src/solvers/flattening/boolbv.h	62.50% <0.00%> (ø)
src/solvers/flattening/boolbv_member.cpp	100.00% <0.00%> (ø)
src/util/bitvector_expr.h	97.34% <0.00%> (+0.09%)	⬆️
src/ansi-c/expr2c.cpp	67.53% <0.00%> (+0.18%)	⬆️
src/solvers/smt2/smt2_conv.cpp	67.61% <0.00%> (+0.21%)	⬆️
src/goto-programs/goto_convert_side_effect.cpp	95.53% <0.00%> (+0.40%)	⬆️
... and 3 more

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