runtime: remove alignment padding in mheap and pageAlloc

runtime: remove alignment padding in mheap and pageAlloc

All subfields use atomic types to ensure alignment, so there's no more
need for these fields.

Change-Id: Iada4253f352a074073ce603f1f6b07cbd5b7c58a
Reviewed-on: https://go-review.googlesource.com/c/go/+/429220
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>