cmd/compile: log large copies on riscv64

Log large copies in the riscv64 compiler.

This was missed in 47ade081, resulting in
the new test added to cmd/compile/internal/logopt failing on riscv64.

Change-Id: I6f763e86f42834148e911d16928f9fbabcfa4290
Reviewed-on: https://go-review.googlesource.com/c/go/+/227804


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>