fix(nvvm): use PassMode::Cast for align(16) ADTs

[Snehal-Reddy]

The NVVM backend was defaulting to PassMode::Direct for all ADTs. On NVPTX, LLVM often aligns generic aggregate types to 8 bytes, even if the Rust type requires 16-byte alignment. This caused the kernel to read arguments from incorrect offsets, leading to crashes.

The Fix:

1. High Alignment (>= 16): For structs requiring 16+ byte alignment, force
   PassMode::Cast using i128 units. This compels LLVM to emit .param .align 16 (or
   .b128) in PTX, guaranteeing the correct ABI contract.
2. Standard Alignment: For other ADTs, explicitly set attrs.pointee_align when using
   PassMode::Direct to ensure LLVM respects the type's alignment attribute.

Fixes #361

[LegNeato]

I'm not really qualified to review this as I am shaky on the LLVM side, but let's land it and see how it goes. Thank you!