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.
We don't support the msvc abi, only mingw.  As for your question, the size of real is the same as on Linux (12 or 16bytes).  IIRC only when targeting bionic does it default to 64bit long doubles.