In turn, isn't LLVM's stack management and argument passing dictated by the hardware's ABI? ARM, for one, publishes its calling convention in AAPCS (ARM Architecture Procedure Call Standard) which establishes the calling convention on the C stack.
If the hardware manufacturer publishes an ABI most people will use it for the sake of convenience and interoperability. But it's not like the ARM police can stop you from using a different calling convention for your language if you want.
ARM and modern x64 have pretty rigorously-defined calling conventions, yeah. x86, on the other hand, has six or seven different ones, and it was up to you as a C programmer to choose which one to use for each given function, by sticking e.g. a __stdcall or a __fastcall in your typespec.
