C library: add imaxabs

[tautschnig]

This is another variant of absolute-value functions.

• 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: 78.35% // Head: 78.35% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (a9d5aa2) compared to base (28b76fa).
Patch coverage: 68.75% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7374      +/-   ##
===========================================
- Coverage    78.35%   78.35%   -0.01%     
===========================================
  Files         1645     1645              
  Lines       190190   190182       -8     
===========================================
- Hits        149031   149017      -14     
- Misses       41159    41165       +6     

Impacted Files	Coverage Δ
unit/solvers/sat/external_sat.cpp	100.00% <ø> (ø)
src/solvers/sat/external_sat.cpp	81.81% <28.57%> (-6.42%)	⬇️
src/ansi-c/c_typecheck_expr.cpp	75.35% <100.00%> (ø)
src/pointer-analysis/value_set.cpp	82.46% <100.00%> (-0.05%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.