bpf2go: emit `structs.HostLayout` into generated structs

[lmb]

We recently made a change to make sure our syscall wrappers continue to work on future versions of go, see #1681. Timo points out that we should do the same for bpf2go:

          Looks good, thanks! We should probably also emit these in bpf2go types.

Originally posted by @ti-mo in #1681 (review)

I think it might make sense to just update GoFormatter to always include HostLayout in structs instead of the hack I did for gentypes.