misc: Move the ExitLogger class definition into misc.cc

misc: Move the ExitLogger class definition into misc.cc

This class isn't referred to outside of misc.hh, and isn't necessarily
useful outside of the particular logging setup implemented in misc.cc.
The Logger class itself is different since it provides a generic
interface that can be used with different logging schemes.

Change-Id: Ibae926fea039d9e3d75a43d97348bc4a3c5d555e
Reviewed-on: https://gem5-review.googlesource.com/6225
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>