i suspect you mean i386, not x32; x32 is an almost unused abi added in 02011 for 64-bit amd and compatible processors, available with gcc -mx32. https://lwn.net/Articles/548838/ talks a bit about its early adoption

it's so little used that i can't find the abi documentation, but gcc -c -mx32 does seem to generate code that passes arguments in registers, not on the stack, contrary to your assertion

edit: after enough digging, i found https://web.archive.org/web/20130902094821/https://sites.goo... linking to https://web.archive.org/web/20201015005434/https://3263a7b2-... but x32-abi-1.0.pdf seems to be missing from the archive and https://web.archive.org/web/*/https://sites.google.com/site/...* doesn't look at all promising