Makefile: Add LTO Optimization

Link Time Optimization (LTO) gives GCC the capability of dumping its internal representation (GIMPLE) to disk, so that all the different compilation units that make up a single executable can be optimized as a single module. This expands the scope of inter-procedural optimizations to encompass the whole program (or, rather, everything that is visible at link time). More about LTO can be found here https://gcc.gnu.org/wiki/LinkTimeOptimization
Thanks @Cl3Kener

Signed-off-by: Lens-F <tnoah12@gmail.com>

Conflicts:
	Makefile
Makefile: use -fgraphite-identity

Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation.

Signed-off-by: Lens-F <tnoah12@gmail.com>