runtime: make addrRange[s] operate on offset addresses

Currently addrRange and addrRanges operate on real addresses. That is,
the addresses they manipulate don't include arenaBaseOffset. When added
to an address, arenaBaseOffset makes the address space appear contiguous
on platforms where the address space is segmented. While this is
generally OK because even those platforms which have a segmented address
space usually don't give addresses in a different segment, today it
causes a mismatch between the scavenger and the rest of the page
allocator. The scavenger scavenges from the highest addresses first, but
only via real address, whereas the page allocator allocates memory in
offset address order.

So this change makes addrRange and addrRanges, i.e. what the scavenger
operates on, use offset addresses. However, lots of the page allocator
relies on an addrRange containing real addresses.

To make this transition less error-prone, this change introduces a new
type, offAddr, whose purpose is to make offset addresses a distinct
type, so any attempt to trivially mix real and offset addresses will
trigger a compilation error.

This change doesn't attempt to use offAddr in all of the runtime; a
follow-up change will look for and catch remaining uses of an offset
address which doesn't use the type.

Updates #35788.

Change-Id: I991d891ac8ace8339ca180daafdf6b261a4d43d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/230717
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/export_test.go

@@ -748,8 +748,8 @@ func (p *PageAlloc) InUse() []AddrRange {
 	ranges := make([]AddrRange, 0, len(p.inUse.ranges))
 	for _, r := range p.inUse.ranges {
 		ranges = append(ranges, AddrRange{
-			Base:  r.base,
-			Limit: r.limit,
+			Base:  r.base.addr(),
+			Limit: r.limit.addr(),
 		})
 	}
 	return ranges