This change scavenges the largest spans before growing the heap for
physical pages to "make up" for the newly-mapped space which,
presumably, will be touched.

In theory, this approach to scavenging helps reduce the RSS of an
application by marking fragments in memory as reclaimable to the OS
more eagerly than before. In practice this may not necessarily be
true, depending on how sysUnused is implemented for each platform.

Fixes #14045.

Change-Id: Iab60790be05935865fc71f793cb9323ab00a18bd
Reviewed-on: https://go-review.googlesource.com/c/139719
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>