SWI only support "SWI $imm", but currently "SWI (Reg)" is also
accepted. This patch fixes it.

And more instruction tests are added to cmd/asm/internal/asm/testdata/arm.s

fixes #20375

Change-Id: Id437d853924a403e41da9b6cbddd20d994b624ff
Reviewed-on: https://go-review.googlesource.com/43552
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>