cmd/asm: on PPC64, fix assembling of FCMPO opcode

cmd/asm: on PPC64, fix assembling of FCMPO opcode

The assembler should pack 3 argument usage of FCMPO similar
to other CMP-like opcodes.

Fixes #67359

Change-Id: Icfb42a67d741431a70dd880806857b4c38f42d62
Reviewed-on: https://go-review.googlesource.com/c/go/+/585640
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>