[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 8c05d67661c966f5130e51ca685b0c70a5a929ff)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/173939
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>