compiler: Fix "power alignment" problems on AIX

[workingjubilee]

Hello, this is a localized rustc-focused fix for the AIX "power alignment" issue. It does not fix upstream because I expect that to be a more annoying experience and would take some time to propagate into the release. I mostly wish to remove the "power alignment" lint so we do not have to work it into updates to the "improper-ctypes" lint, but it feels wrong to do so without actually fixing the codegen issue, especially since it's such a small change.

cc @daltenty @gilamn5tr @mustartt @amy-kwan Can you confirm whether this change allows rustc to do FFI correctly with C code compiled using the default AIX ABI?

[rustbot]

r? @wesleywiser

rustbot has assigned @wesleywiser.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[RalfJung]

It does not fix upstream

I assume you are referring to llvm/llvm-project#133599 here? Always good to leave some cross-references. :)

Please also add comments in the code referencing that (both in the aix target triple, and the special exception for the data layout consistency check). Right now, just looking at the code after applying this diff, one would have a very hard time figuring out what happens.

[workingjubilee]

aye aye capitan

[RalfJung]

As discussed on IRLO, as-is this patch would make some types more correct and others less correct. Hard to say whether that's overall a net positive...