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>

src/runtime/mheap.go

@@ -65,8 +65,6 @@ type mheap struct {
 	// could self-deadlock if its stack grows with the lock held.
 	lock mutex
 
-	_ uint32 // 8-byte align pages so its alignment is consistent with tests.
-
 	pages pageAlloc // page allocation data structure
 
 	sweepgen uint32 // sweep generation, see comment in mspan; written during STW
@@ -84,8 +82,6 @@ type mheap struct {
 	// access (since that may free the backing store).
 	allspans []*mspan // all spans out there
 
-	// _ uint32 // align uint64 fields on 32-bit for atomics
-
 	// Proportional sweep
 	//
 	// These parameters represent a linear function from gcController.heapLive
@@ -191,8 +187,6 @@ type mheap struct {
 		base, end uintptr
 	}
 
-	_ uint32 // ensure 64-bit alignment of central
-
 	// central free lists for small size classes.
 	// the padding makes sure that the mcentrals are
 	// spaced CacheLinePadSize bytes apart, so that each mcentral.lock

src/runtime/mpagealloc.go

@@ -267,8 +267,6 @@ type pageAlloc struct {
 	// All access is protected by the mheapLock.
 	inUse addrRanges
 
-	_ uint32 // Align scav so it's easier to reason about alignment within scav.
-
 	// scav stores the scavenger state.
 	scav struct {
 		// index is an efficient index of chunks that have pages available to