misc: Rename Debug namespace as debug

As part of recent decisions regarding namespace
naming conventions, all namespaces will be changed
to snake case.

gem5::Debug became gem5::debug.

Change-Id: Ic04606baab3317d2e58ab3ca9b37fc201c406ee8
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47305
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
53 files changed