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 register.

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:
fe2119a7b0/clang/lib/CodeGen/Targets/PPC.cpp (L878-L884)
fe2119a7b0/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.

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

compiler/rustc_codegen_llvm/src/abi.rs

@@ -150,7 +150,7 @@ impl LlvmType for CastTarget {
         // Simplify to a single unit or an array if there's no prefix.
         // This produces the same layout, but using a simpler type.
         if self.prefix.iter().all(|x| x.is_none()) {
-            if rest_count == 1 {
+            if rest_count == 1 && !self.rest.force_array {
                 return rest_ll_unit;
             }