Fixed a build error on non-win32 platfroms

Due to dl* functions usage in Core/HW/atrac3plus.cpp ([here](https://github.com/hrydgard/ppsspp/blob/master/Core/HW/atrac3plus.cpp#L4)) on non-windows
platforms, Core library needs to be linked with -ldl flag on these
platforms.

Added a CoreExtraLibs variable in CMakeLists.txt to handle all
platform-dependent dependencies of Core, and added dl to this variable
on non-win32 platforms.