Commit 759be3dad9 bumped our Go
requirement to 1.18 which allows us to drop support for old-style
`+build` tags. This change runs `go fix ./...` to remove them, and
updates some remaining code generators that produced `+build` lines.
This fixes a bug in argument size calculation in the case where the function
has no return values. Previously it was padding the argument struct to max
alignment, but this only happens if there are return values following.
Updates #191