scons: When importing SimObjects, use a copy of globals().

Use a copy of globals when importing SimObjects, instead of the real
deal that the SConscript runs under. This avoids polluting the
SConscript namespace.

Change-Id: I689d693d93cc8998be0967ec2b0c58f5851d39cf
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49407
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>