MSVC currently doesn't support 80 bits long double. But ICC does support
it on Windows. Besides, there're also some users asked for this feature.
We can find the discussions from stackoverflow, msdn etc.
Given Clang has already support -mlong-double-80, extending it to
support for Windows seems worthwhile.
If GCC aligns f80 to 16 bytes, we might as well make the change here and share it with the mingw target.