Add c_long_double for interacting with C code.

Based on a recent newsgroup discussion, I think that it's clear that we
need an alias for C's long double, so here it is. I think that I got it
right for x86, and x86_64, but I have no idea what it should be for
other platforms.
> I think that that comes down to a question of what gcc does with the weirder architecture

It will do the same across all architectures. The long_double type is set-up as part of common types in the middle-end.  Weird backend architectures will invent their own type names (eg: __float128).