fastmodel: Determine what space to use for breakpoints dynamically.

fastmodel: Determine what space to use for breakpoints dynamically.

This was hardcoded as 5, but should be determined based on the memory
space IDs the fast model returns. What we do now is have a specific
override for ARM (perhaps conceptually the A76) which looks for an
address space called "Current" which seems to work well.

It's possible that the appropriate address space for a different model
might have a different number, or even a different name. This may need
to be further specialized/parameterized in those cases.

Change-Id: Ie1ef99675fd9bccab50b7fc7add16b82a93bd60b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22143
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>