C model library: Support ARM64 va_list types

[tautschnig]

GCC/ARM appears to uses a struct to represent variadic arguments (where the first component is a pointer to the stack of arguments). Taking the pointer offset requires looking at the first element of this struct, which we do by taking the address, casting to a pointer, and dereferencing.

Fixes: #7423

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[codecov]

Codecov Report

Base: 78.50% // Head: 78.50% // No change to project coverage 👍

Coverage data is based on head (4d0164f) compared to base (3f578e1).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7521   +/-   ##
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  Coverage    78.50%   78.50%           
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  Files         1663     1663           
  Lines       191297   191297           
========================================
  Hits        150174   150174           
  Misses       41123    41123           

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