scons: Keep the list of all CPU models in a construction variable.
scons: Keep the list of all CPU models in a construction variable.

This relaxes ordering around CPU model declaration.

Change-Id: Ie5f8da1903a883bd4d98800dfbef7fd913021f77
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48964
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>

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