arch-arm: fix the aarch64 GDB stub

arch-arm: fix the aarch64 GDB stub

The main change is to remove vector registers from the GDB stub.

Those registers were intended for SVE, which is a new architecture feature
and not yet treated by default on the GDB present in Ubuntu 18.04, and
possibly not even on GDB master.

As a result, aarch64 GDB stub connections would fail with:

Remote 'g' packet reply is too long

The correct way to support those registers is to send XML GDB target
description files to the client. This feature is not yet available for
any architecture, and should be implemented in future patches.

Other smaller fixes are:

* cpsr is uint32_t in aarch64 as well as arm
* use M5_ATTR_PACKED on the register structs since they are being cast and
  sent as byte arrays

Change-Id: I77cd8a98e322ecc60799e5b11fe5cd414d893cc7
Reviewed-on: https://gem5-review.googlesource.com/c/14495
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>