makefile: don't set TARGETOS while setting OSD, it will be set later
makefile: fix some comments
makefile: fix default NATIVE_OBJ
Run build tools based on build OS
Add CROSS_PREFIX to make cross-compiling easier

For example, to cross-compile for 64-bit windows:
make buildtools
make TARGETOS=win32 OSD=windows NATIVE_OBJ=obj/sdl64 \
     CROSS_BUILD=1 CROSS_PREFIX=x86_64-w64-mingw32-
Fix crash on fatalerror from winwindow_video_window_create()