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 architectures. If we _know_ that it will always be `real` regardless of the architecture, then that's fine, but gcc and llvm support a lot of different architectures, many of which do things differently from x86.