Added Sys_FreeClipboardData(char*) so I don't have to copy the string
from SDL_GetClipboardText() into a Mem_Alloc() buffer, but can just
do the right thing per platform, which in case of POSIX/SDL2 is
SDL_free().
SDL1.2 doesn't have clipboard support, otherwise I'd have removed all
platform-specific implementations and used SDL_Get/SetClipboardText()
everywhere (IIRC AROS only supports SDL1.2?)