Rollup merge of #122781 - nikic:ppc-abi-fix, r=cuviper

Fix argument ABI for overaligned structs on ppc64le

When passing a 16 (or higher) aligned struct by value on ppc64le, it needs to be passed as an array of `i128` rather than an array of `i64`. This will force the use of an even starting doubleword.

For the case of a 16 byte struct with alignment 16 it is important that `[1 x i128]` is used instead of `i128` -- apparently, the latter will get treated similarly to `[2 x i64]`, not exhibiting the correct ABI. Add a `force_array` flag to `Uniform` to support this.

The relevant clang code can be found here:
https://github.com/llvm/llvm-project/blob/fe2119a7b08b6e468b2a67768904ea85b1bf0a45/clang/lib/CodeGen/Targets/PPC.cpp#L878-L884
https://github.com/llvm/llvm-project/blob/fe2119a7b08b6e468b2a67768904ea85b1bf0a45/clang/lib/CodeGen/Targets/PPC.cpp#L780-L784

I think the corresponding psABI wording is this:

> Fixed size aggregates and unions passed by value are mapped to as
> many doublewords of the parameter save area as the value uses in
> memory. Aggregrates and unions are aligned according to their
> alignment requirements. This may result in doublewords being
> skipped for alignment.

In particular the last sentence. Though I didn't find any wording for Clang's behavior of clamping the alignment to 16.

Fixes https://github.com/rust-lang/rust/issues/122767.

r? `@cuviper`

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