cmd/compile: remove backend's "scratch mem" support

This CL rebases CL 273987 on top of master with @mdempsky's permission.

The last (only?) use for this feature was 387 support, which was
removed in golang.org/cl/258957.

Change-Id: I4f79fee8d0c336c9b6082bcd5eb6ece52c032dc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/292893


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>