go/token, syscall: annotate if blocks that defeat vet's unreachable pass

go/token, syscall: annotate if blocks that defeat vet's unreachable pass

Since putting code in an "if true" block is unusual, make it easier
for readers to understand the purpose of doing this.

For #73998.

Change-Id: I3fd8d65130211c7c01d424366a3c662482d80add
Reviewed-on: https://go-review.googlesource.com/c/go/+/702416
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>