scons: Make the SimObject list from the 'gem5 lib' tag.
scons: Make the SimObject list from the 'gem5 lib' tag.

Only include SimObject files which match the gem5 lib tag. This way we
can declare SimObjects, and then filter them out based on tags.

Change-Id: I0aca1ef830bcc7beaee80c54d58ba8a188968491
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50331
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5