cmd/internal/obj/arm64: fix encoding of condition

cmd/internal/obj/arm64: fix encoding of condition

The current code treats condition as special register and write
its raw data directly into instruction.

The fix converts the raw data into correct condition encoding.
Also fix the operand catogery of FCCMP.

Add tests to cover all cases.

Change-Id: Ib194041bd9017dd0edbc241564fe983082ac616b
Reviewed-on: https://go-review.googlesource.com/41511
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>