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.
But if all of the Posix platforms are the same, why list them all separately? I makes sense to me to list them separately when they're different, but if they're not different, it's just extra code for no benefit.