preconditions for abs, labs, llabs, imaxabs

[kroening]

C99 states that the behavior of abs, labs, llabs, imaxabs is undefined when the result of computing the absolute value is not representable. This commit adds preconditions to the library models that catch this case.

It is unclear whether the same preconditions should be added for the GCC builtins.

• 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.
• 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

Patch coverage has no change and project coverage change: +0.20% 🎉

Comparison is base (63b8b71) 78.13% compared to head (d1dc64f) 78.34%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7825      +/-   ##
===========================================
+ Coverage    78.13%   78.34%   +0.20%     
===========================================
  Files         1698     1698              
  Lines       194027   194027              
===========================================
+ Hits        151600   152004     +404     
+ Misses       42427    42023     -404     

see 53 files with indirect coverage changes

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