Add support for undocumented varargs intrinsics

[cirras]

This PR adds support for 4 undocumented intrinsics that are usable within varargs routines.

Signatures for these intrinsics can be found in System:

  // procedure VarArgStart(var ArgList: TVarArgList);
  // function  VarArgGetValue(var ArgList: TVarArgList; ArgType: Type): ArgType;
  // procedure VarArgCopy(var DestArgList, SrcArgList: TVarArgList);
  // procedure VarArgEnd(var ArgList: TVarArgList);

We can't easily inject intrinsics with specific concrete types like System.TVarArgList, so I've opted for the "good enough" solution of using untyped parameters for these. I can't see any obvious way for this to result in a less inaccurate analysis (on source code that successfully compiles).

See also

• How to create a Delphi variadic method similar to Write/Writeln without requiring brackets for arguments?
• [fpc-devel] C-block reference syntax (blocker for 3.2)

[fourls]

Looks good! Now I can happily scan all my most upsetting Win64 variadics!!