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>

src/syscall/mksyscall.pl

@@ -350,7 +350,7 @@ while(<>) {
 			$text .= "//go:linkname $funcname $funcname\n";
 			# Tell the linker that funcname can be found in libSystem using varname without the libc_ prefix.
 			my $basename = substr $funcname, 5;
-			$text .= "//go:cgo_import_dynamic $funcname $basename \"/usr/lib/libSystem.B.dylib\"\n";
+			$text .= "//go:cgo_import_dynamic $funcname $basename \"/usr/lib/libSystem.B.dylib\"\n\n";
 		}
 	}
 }