syscall: make //sys lines not doc comments

syscall: make //sys lines not doc comments

If they are doc comments then gofmt will put a space between // and sys.
Most of syscall was already this way, following CL 7324056 (in 2013).
These were not.

Change-Id: Ie6ebf82809c199d0d06b87c86045bbb62b687d5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/407136
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>