__intN patch 3/5: main __int128 -> __intN conversion.

[DJ Delorie]

> ../../gcc/gcc/config/rs6000/rs6000-c.c:237:24: error: ‘RID_INT128’ was not declared in this scope

Two options:

1. If you know the RS6000 will never have any __intN other than
   __int128, just use RID_INT_N_0, although this is a hack it will
   work as long as there *is* an __int128 for RS6000.

2. Alternately, you need to check all entries in the __int<N> array
   for proper size, which is more correct but more complex.

Would you like me to work on the second option, or would you prefer to
tackle this yourself?