cmd/internal/obj/arm64: disable the pre and post index formats for pseudo registers

When using the FP or SP pseudo-register to load or store, pre-index and post-index formats
are not supported because the RSP and pseudo registers are not allowed to be modified in this
way. This CL deletes the related entries in optab and adds a few test cases.

Change-Id: Ie30d27d0e7b959242f0e6298b950489669d07989
Reviewed-on: https://go-review.googlesource.com/c/go/+/342770


Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: eric fang <eric.fang@arm.com>
Trust: Michael Knyszek <mknyszek@google.com>