x86 long double coming

I think this has come far enough that I can turn the switch in llvm-gcc that
makes long doubles 80 bits on x86. There are probably bugs, but quite
a few things work, including all the long doubles in the test suite.

This is an ABI change, so I'm giving notice. I'm planning to do this sometime
tomorrow morning California time, say 20 hours from now. Let me know if you
have objections.