cmd/asm: disable scaled register format for arm64

Arm64 doesn't have scaled register format, such as (R1*2), (R1)(R2*3),
but currently the assembler doesn't report an error for such kind of
instruction operand format. This CL disables the scaled register
operand format for arm64 and reports an error if this kind of instruction
format is seen.
With this CL, the assembler won't print (R1)(R2) as (R1)(R2*1), so that
we can make the assembly test simpler.

Change-Id: I6d7569065597215be4c767032a63648d2ad16fed
Reviewed-on: https://go-review.googlesource.com/c/go/+/289589


Trust: eric fang <eric.fang@arm.com>
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>