cpu: Make getIsaPtr return a BaseISA pointer.

cpu: Make getIsaPtr return a BaseISA pointer.

This isolates the architecture specific ISA types a little bit, and
means that ThreadContexts don't *have* to find an architecture specific
class to return, even if they don't naturally have one lying around.

Change-Id: Ide10b5d945ec6076947b2ccdea87c86e96e40857
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25008
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>