base: Cleanup debug flags API

base: Cleanup debug flags API

The debug flags API has a couple of quirks that should be cleaned
up. Specifically:

 * Only CompoundFlag should expose a list of children.
 * The global enable flag is just called "active", this isn't very
   descriptive.
 * Only SimpleFlag exposed a status member. This should be in the base
   class to make the API symmetric.
 * Flag::Sync() is an implementation detail and needs to be protected.

Change-Id: I4d7fd32c80891191aa04f0bd0c334c8cf8d372f5
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34118
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>