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 simplified it by having a single static assert at the bottom which checks that `c_long_double` has been defined. So, it's only in one place rather than having to have an `else` on each of the version-else chains.