syscall: add empty line before marker comments in zsyscall_darwin_*.go

syscall: add empty line before marker comments in zsyscall_darwin_*.go

This was spotted during the review of the corresponding CL 154179 for
x/sys/unix. Let's change it in syscall as well to be consistent.

Change-Id: I33f25db1f6ba941b694c2aa276336448cc2b9b51
Reviewed-on: https://go-review.googlesource.com/c/154719
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>