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

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

The ssa compiler uses the duffcopy and duffzero functions,
which rely on the MOVUPS instructions.

However, this doesn't work on Plan 9, since floating point
operations are not allowed in the note handler.

This change disables the use of duffcopy and duffzero
on Plan 9 in the ssa compiler.

Updates #14605.

Change-Id: I017f8ff83de00eabaf7e146b4344a863db1dfddc
Reviewed-on: https://go-review.googlesource.com/20171
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>