Use C++ style comments consistently in SDL source code

Implemented using this script:

find . -type f -exec sed -i'' -e 's,/\* *\([^*]*\)\*/ *$,// \1,' -e 's, \+$,,' {} \;
git checkout \
    core/linux/SDL_evdev_kbd_default_keymap.h \
    events/imKStoUCS.* \
    hidapi \
    joystick/controller_type.c \
    joystick/controller_type.h \
    joystick/hidapi/steam/controller_constants.h \
    joystick/hidapi/steam/controller_structs.h \
    joystick/SDL_gamepad_db.h \
    libm \
    render/*/*Shader*.h \
    render/vitagxm/SDL_render_vita_gxm_shaders.h \
    render/metal/SDL_shaders_metal_*.h \
    stdlib/SDL_malloc.c \
    stdlib/SDL_qsort.c \
    stdlib/SDL_strtokr.c \
    test/ \
    video/directx/SDL_d3d12_xbox_cmacros.h \
    video/directx/d3d12.h \
    video/directx/d3d12sdklayers.h \
    video/khronos \
    video/x11/edid-parse.c \
    video/x11/xsettings-client.* \
    video/yuv2rgb
sed -i'' -e 's,/\* *\([^*]*\)\*/ *$,// \1,' -e 's, \+$,,' hidapi/SDL_hidapi.c

src/filesystem/emscripten/SDL_sysfilesystem.c

@@ -23,7 +23,7 @@
 #ifdef SDL_FILESYSTEM_EMSCRIPTEN
 
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
-/* System dependent filesystem routines                                */
+// System dependent filesystem routines
 
 #include "../SDL_sysfilesystem.h"
 
@@ -113,4 +113,4 @@ char *SDL_SYS_GetUserFolder(SDL_Folder folder)
     return retval;
 }
 
-#endif /* SDL_FILESYSTEM_EMSCRIPTEN */
+#endif // SDL_FILESYSTEM_EMSCRIPTEN