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.
Because it's the fact they're the same has nothing to do with the fact they're all posix.
```d
version( OperatingSystemWithNameContainingTheLetterW)
     alias double c_long_double;
else
     alias real c_long_double;
```