runtime: convert timer0When/timerModifiedEarliest to atomic.Int64

runtime: convert timer0When/timerModifiedEarliest to atomic.Int64

So they match with when/nextwhen fields of timer struct.

Updates #53821

Change-Id: Iad0cceb129796745774facfbbfe5756df3a320b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/423117
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>