runtime: remove pageAlloc.scav padding for atomic field alignment

runtime: remove pageAlloc.scav padding for atomic field alignment

CL 404096 makes atomic.Int64 8 bytes aligned everywhere.

Change-Id: I5a676f646260d6391bb071f9376cbdb1553e6e6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/424925
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>