mem-cache: Fix CacheSet memory leak

mem-cache: Fix CacheSet memory leak

CacheSet blocks were being allocated but never freed.
Used vector to avoid using pure C array.

Change-Id: I6f32fa5a305ff4e1d7602535026c1396764102ed
Reviewed-on: https://gem5-review.googlesource.com/8603
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>