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 <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Auto-Submit: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go