[NVPTX] Use the mask() operator to initialize packed structs with pointers

The current implementation assumes that all pointers used in the
initialization of an aggregate are aligned according to the pointer size
of the target; that might not be so if the object is packed. In that
case, an array of .u8 should be used and pointers should be decorated
with the mask() operator.

The operator was introduced in PTX ISA 7.1, so an error is issued if the
case is detected for an earlier version.

Differential Revision: https://reviews.llvm.org/D127504