runtime: don't use duffcopy on Plan 9

runtime: don't use duffcopy on Plan 9

In CL 14836, the implementation of duffcopy on amd64
was changed to replace the use of the MOVQ instructions
by MOVUPS.

However, it broke the build on plan9/amd64, since
Plan 9 doesn't allow floating point in note handler.

This change disables the use of duffcopy on Plan 9.

Fixes #12829.

Change-Id: Ifd5b17b17977a1b631b16c3dfe2dc7ab4ad00507
Reviewed-on: https://go-review.googlesource.com/15421
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>