Aligned structs in packed structs

[mahkoh]

The packed struct cgw_frame_mod in the linux kernel api contains the aligned struct can_frame. This cannot be written in Rust because

packed type cannot transitively contain a `#[repr(align)]` type

@alexcrichton, @nikomatsakis, and @retep998 discussed this issue in #1358 and the decision was to disallow this.

But this restriction is unnecessary for repr(C) types. For repr(C) types, the layout is the layout defined by the C target environment. That environment is determined by the target 4-tuple.