First step to port ROOT on Windows 64 bit (#7759)

* First step to port ROOT on Windows 64 bit

 - Introduce the Win64 option and flags in the CMake build system
 - Introduce the new `Longptr_t` and `ULongptr_t` types to hold a pointer (4 bytes on Win32 and 8 bytes on Win64), aimed to replace the `Long_t` and `ULong_t` used to cast pointers, and being both 4 bytes on Windows. Their values will not change on other platforms (`Long_t` and `ULong_t`) for backward compatibility
 - Add the 64 bit export symbols in `core/metacling/src/CMakeLists.txt`
 - Add the 64 bit version of lzma (xz-5.2.4-win64.tar.gz)

* Add forgotten semicolons