base: Move Stats::Info functions to its own source file

base: Move Stats::Info functions to its own source file

Move information needed by Stats::Info and its derived classes
from base/statistics.cc to its own source file.

Create a SConscript in the stats sub-dir to start clustering
stats related files.

Change-Id: I1e5e828c7814748c2582755f664550241caf860e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25424
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>