Where is the right place to set the array of "this __intN mode is enabled" flags? I initially set it in tree.c where __int128 is set up, but that happens *after* c_parse_init() needs the flag to set up the RID_* keywords for them.
Alternately, should I be calling targetm.scalar_mode_supported_p() all over the place? We'd need to be careful to not call that too early too...
