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 +/- ##
========================================
Coverage 79.66% 79.66% ========================================
Files 1682 1682 Lines 195377 195377 ========================================
Hits 155641 155641 Misses 39736 39736 

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