[release-branch.go1.12] runtime: make mTreap.find actually find the best fit

This change modifies the implementation of mTreap.find to find the
best-fit span with the lowest possible base address.

Fixes #31677.

Change-Id: Ib4bda0f85d7d0590326f939a243a6e4665f37d3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/173479


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
(cherry picked from commit 8c05d676)
Reviewed-on: https://go-review.googlesource.com/c/go/+/173939


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>