stdio: add __vdprintf_chk

Daniël van de Giessenrequested to merge
DvdGiessen/gcompat:vdprintf_chk into current

Implements __vdprintf_chk based on !20 (merged) which added __dprintf_chk.

  • As noted in that previous MR the flags parameter is ignored.
  • Added a not-NULL assert on format, similar to other functions, which will also apply to __dprintf_chk.

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