mem-cache: Vectorize C arrays in BaseSetAssoc.

Transform BaseSetAssoc's arrays into C++ vectors to avoid unnecessary
resource management.

Change-Id: I656f42f29e5f9589eba491b410ca1df5a64f2f34
Reviewed-on: https://gem5-review.googlesource.com/8621
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>