Add CMake option to enable LTO for AtlasGeant4

Use of LTO in Athena builds is not currently possible if the GCC checker
plugins are in use. These are not yet LTO-aware, leading to build
errors. Disabling these globally or per-LTO-using-package is not
feasible as it could lead to the introduction of bad code.

Whilst the plugins are made LTO-aware, introduce CMake option in Athena
and AthSimulation for enabling LTO builds of the AtlasGeant4 "big"
library. Set to OFF by default so that "normal" builds are unaffected.
Builds wanting LTO can set the option to ON, but must also set
ATLAS_USE_GCC_CHECKERS to OFF in this case.