cmd/compile: don't use duffzero on Plan 9

cmd/compile: don't use duffzero on Plan 9

In CL 14408, the implementation of duffzero 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 duffzero on Plan 9.
We also take care to not use the MOVUPS instruction.

Fixes #14471.

Change-Id: I8277b485dfe65a68d7d8338e52a048c5d45069bf
Reviewed-on: https://go-review.googlesource.com/19890
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>