runtime: throw if scavenge necessary during coalescing

Currently when coalescing if two adjacent spans are scavenged, we
subtract their sizes from memstats and re-scavenge the new combined
span. This is wasteful however, since the realignment semantics make
this case of having to re-scavenge impossible.

In realign() inside of coalesce(), there was also a bug: on systems
where physPageSize > pageSize, we wouldn't realign because a condition
had the wrong sign. This wasteful re-scavenging has been masking this
bug this whole time. So, this change fixes that first.

Then this change gets rid of the needsScavenge logic and instead checks
explicitly for the possibility of unscavenged pages near the physical
page boundary. If the possibility exists, it throws. The intent of
throwing here is to catch changes to the runtime which cause this
invariant to no longer hold, at which point it would likely be
appropriate to scavenge the additional pages (and only the additional
pages) at that point.

Change-Id: I185e3d7b53e36e90cf9ace5fa297a9e8008d75f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/158377


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>