runtime: remove p padding field for atomic fields alignment

runtime: remove p padding field for atomic fields alignment

CL 424396 and CL 424397 changed timer0When/timerModifiedEarliest to
atomic.Uint64, just they're guaranted to have 64-bit alignment.

Change-Id: Idaff1059da2aac84520b9b0e34f9721a74dbba5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/424794
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>