[NVPTX] Use align attribute for kernel pointer arg alignment

Instead of determining the alignment based on the pointer element
type (which is incompatible with opaque pointers), make use of
alignment annotations added by the frontend.

In particular, clang will add alignment attributes to OpenCL kernels
since D118894. Other frontends might need to be adjusted to add
the attribute as well.

Differential Revision: https://reviews.llvm.org/D119247