Support __builtin_dynamic_object_size

[tautschnig]

Use the same approximation of the behaviour as we already did for __builtin_object_size.

Fixes: #8116

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (69bb2b6) 79.09% compared to head (873727c) 79.09%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8156      +/-   ##
===========================================
- Coverage    79.09%   79.09%   -0.01%     
===========================================
  Files         1695     1695              
  Lines       196583   196585       +2     
===========================================
- Hits        155496   155495       -1     
- Misses       41087    41090       +3     

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[peterschrammel]

❓Just out of interest, would it actually be possible to return the precise result?

[tautschnig]

❓Just out of interest, would it actually be possible to return the precise result?

I believe "precise" would mean returning exactly what (some version of) gcc returns for a given program. We could make some assumptions as to what "known [to GCC] at compile time," but I'm not sure whether this would be of much help.