mem-ruby: Enable set size increase

mem-ruby: Enable set size increase

Add NUMBER_BITS_PER_SET environment variable to control
the size of the bitmask in Set.hh (default=64).
Necessary for configs which require >64 instances of a given
machine type. This can be set in the build_opts file, e.g.
by adding the following line:
NUMBER_BITS_PER_SET = <number>

Change-Id: I314a3cadca8ce975fcf4a60d9022494751688e88
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18968
Reviewed-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>