cmd/internal/obj/arm64: add error report for invalid base register

cmd/internal/obj/arm64: add error report for invalid base register

The current assembler accepts the non-integer register as the base register,
which should be an illegal combination.

Add the test cases.

Change-Id: Ia21596bbb5b1e212e34bd3a170748ae788860422
Reviewed-on: https://go-review.googlesource.com/134575
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>