runtime: avoid bound check in freebsd binuptime

runtime: avoid bound check in freebsd binuptime

Fixes #76062

Change-Id: I683c1232aaeac12b0b3688472bb277adb95ad542
Reviewed-on: https://go-review.googlesource.com/c/go/+/715180
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>