WinRT build fix for ARM platforms

The _xgetbv intrinsic was being used in ARM builds of SDL/WinRT, which was
leading to linker errors.  This commit limits _xgetbv use to the platforms on
which it is available, x86 and x64.
Removed 42 from README.
Updated README name in header.
Fixed doxygen warning and markdown formatting.
Added javadoc comment for consistency.
Fixed typo in source comment.
Added an entry for the new Steam controller XInput emulation mode
Implemented SDL_GetPrefPath() on Android - it returns the path used by SDL_AndroidGetInternalStoragePath()
Fixed doxygen warnings and markdown formatting.
Fixed warning about implicit boxing to Java Object.
Fixed warnings about unused local variables.
Take advantage of GL_ARB_texture_non_power_of_two when it's available