Optimized VS2015 x64 compilation flags

[akaStiX]

Default compilation flags for Release builds are not optimal, so every time we update PhysX I have to manually fix flags for all target platforms. This pull request enables Link Time Code Generation and dead code\data elimination for VS2015 x64 target. It's a partial pull request, if you are willing to accept it - I'll go through every other VS project, will fix flags and whitespaces and will create a new pull request. The same issue exists in console projects, how do I create pull requests for XB1 and PS4?