It is my understanding that the cruft is microsoft-specific: other systems use cdecl, fastcall and stdcall are related to Windows APIs and techs. And even though AMD published a call convention as part of AMD64, Microsoft ignored it in favour of their own fastcall-derived CC[0], and have added a new one since (vectorcall[1])
[0] https://msdn.microsoft.com/en-us/library/ms235286.aspx
[1] https://msdn.microsoft.com/en-us/library/dn375768.aspx