Add test for rust-lang/rust#122767

[cuviper]

In rust-lang/rust#122767, ppc64le was found to have a mismatched ABI on this struct:

#[repr(C)]
#[repr(align(16))]
pub struct TwoU64s {
    pub a: u64,
    pub b: u64,
}

Rust was passing this like [2 x i64], while Clang passes [1 x i128] -- notably, it's not bare i128 either.

[Gankra]

I believe this would be TwoU64s.procgen.kdl:

@align 16
struct "TwoU64s" {
    a "u64"
    b "u64"
}

Although see #15 (comment) for caveats on align not being fully implemented/tested yet.

[Gankra]

Filed #26 for the main blocker