scons: Add an "All" compound debug flag

scons: Add an "All" compound debug flag

Add an "All" compound debug flag, which encapsulates all
debug flags.

Since this is the broadest compound flag, allowing users
to include it would imply in extremely generic includes.
Moreover, it is highly unlikely that any correct C++ code
would ever use all debug flags. Therefore, a header file
for this flag is not generated to force users to directly
include only the debug flags they need.

Change-Id: If40f2f708be1495fa2b2380266164d5d44d7cffa
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39077
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@huawei.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>