syscall: follow convention for generated code comment

syscall: follow convention for generated code comment

Follow the convention (https://golang.org/s/generatedcode) for generated
code as if we would regenerate them using mkall.sh. This also drops the
superfluous //go:build tags which are already implied by the file names.

Change-Id: Ic97c6ba9d707d347f06a7d6c1dd07b556d82c882
Reviewed-on: https://go-review.googlesource.com/c/go/+/611115
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>