arm: Make the semihosting implementation use GuestABI.

arm: Make the semihosting implementation use GuestABI.

Remove the ability to not have an implementation for a semihosting call
in 32 or 64 bit mode since that was not actually being used. It can be
reintroduced if needed in the future.

Turn the physProxy helper function into a static function which
maintains a single secure port proxy. That makes the proxy available
outside of the ArmSemihosting class itself.

Change-Id: Ie99e7d79c08c039384250fab0c98117554c93128
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25946
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>