syscall: hide internal comment from Syscall documentation

syscall: hide internal comment from Syscall documentation

There's a comment on the Syscall function that's supposed to be an
internal implementation note, but since it's not separated from the
function definition, it appears in godoc. Add a blank line to prevent
this.

Change-Id: Iba307f1cc3844689ec3c6d82c21d441852e35bca
Reviewed-on: https://go-review.googlesource.com/c/go/+/236561
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>