C library: model __builtin_powi{,f,l}

[tautschnig]

These GCC built-ins are modelled by invoking their non-integer counterparts pow{,f,l}.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9c7bccc) 79.66% compared to head (e26fc74) 79.66%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8192   +/-   ##
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  Coverage    79.66%   79.66%           
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  Files         1682     1682           
  Lines       195377   195377           
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  Hits        155641   155641           
  Misses       39736    39736           

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