scons: Get rid of the all_isa_deps variable.

scons: Get rid of the all_isa_deps variable.

This value can be computed more directly and more locally near where it's
used.

Change-Id: Ib5f45015494a6c8033ce0ac4b8931688f37492c8
Reviewed-on: https://gem5-review.googlesource.com/2982
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>