@mcc The official tools Intel provides are the C intrinsics - and they are of course C syntax, so have no bearing on the assembly.
So yeah, my recollection is we picked what seemed sensible and went with it. BUT - that was just for the purposes of ISA documentation - there was no hard link to the actual syntax accepted by the assemblers (dramatically so in the case of AT&T syntax).
So it really does seem like a thing nobody owns, except for each specific tool vendor!