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/15421Reviewed-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>

src/cmd/compile/internal/amd64/cgen.go

@@ -80,7 +80,7 @@ func blockcopy(n, ns *gc.Node, osrc, odst, w int64) {
 		gins(x86.ACLD, nil, nil)
 	} else {
 		// normal direction
-		if q > 128 || (gc.Nacl && q >= 4) {
+		if q > 128 || (gc.Nacl && q >= 4) || (obj.Getgoos() == "plan9" && q >= 4) {
 			gconreg(movptr, q, x86.REG_CX)
 			gins(x86.AREP, nil, nil)   // repeat
 			gins(x86.AMOVSQ, nil, nil) // MOVQ *(SI)+,*(DI)+