So, I've found out that I was wrong, but I was wrong in a way that is worse for everyone, sortof: The AIX "alignment rule" seems to actually just be an extremely convoluted way of saying "this is sometimes overaligned when it is put on the stack". And this is mostly irrelevant: ABIs cannot resist having ad-hoc modifications to how stack passing works, so they all have some ad hoc rule or another, and the stack layout is otherwise a purely internal implementation detail.
...However, the reason was because I was going off what I was being told about how the alignment should work, and despite asking many, many questions, I didn't realize that the alignment should in fact be only 4 for f64 AKA double. I was incorrect because LLVM does not know the alignment of the types for AIX, and because GCC and clang conflate, often, discussion of "required alignment" and "preferred alignment". Instead, only clang overrides the data layout for AIX, meaning that other frontends for LLVM, like, say, flang or other Fortran compilers, or in this case rustc, don't work for AIX, because the C compiler has been modified to have an ABI that disagrees with every other LLVM front end. The alignment for f64 that rustc currently thinks AIX has is 8.
If the rule implemented in clang, as it currently is, happens to be correct? Then AIX actually has zero modifications from repr(C) and this has been much ado about nothing.