arch-arm: Add assertions when extracting an ArmSystem from a TC

arch-arm: Add assertions when extracting an ArmSystem from a TC

We sometimes need to cast the System pointer stored in a
ThreadContext to an ArmSystem pointer to query global
system setting.
Add an assertion to make sure that the cast resulted in a
valid pointer.

Change-Id: Id382d0c1dceefee8f74d070c205c7b43b83ab215
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/6161
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>