[Clang][Cygwin] Cygwin x86_64 should accept __stdcall (#158385)
Cygwin should support calling convention attributes `__cdecl`,
`__stdcall`, `__thiscall`, and `__fastcall`, even though they have no
effect in x86_64, as done in MinGW.
Originally reported in
https://cygwin.com/pipermail/cygwin/2025-September/258782.html
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Co-authored-by: Jeremy Drake <github@jdrake.com>