Added CMake build system (to facilitate Win64 builds)

Win32 build and tests work under CMake, however I haven't added
install code yet. Win64 build fails due to chkstk.S failing to
assemble.
64-bit tests now pass (well, nearly).

tcctest1-3 fail, but this appears to be due to bugs in GCC rather than TCC
(from manual inspection of the output).