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.
> it's just extra code for no benefit.

The benefit is when support for a new Posix platform is added, where real != long double, the code will not silently break.