runtime: remove gcCPULimiterState padding for atomic fields alignment

runtime: remove gcCPULimiterState padding for atomic fields alignment

assistTimePool and lastUpdate are now atomic.Int64, so they are
guaranteed to have 64-bit alignment, even on 32-bit platforms.

Change-Id: Ib6062a47c3a92d46d43899e1ae9d119e7f5b8bb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/425460
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>