FP modes are messed up after exceptions on Windows 64

[Stas Boukarev]

When foreign code raises an FP exception it clears the current traps, so running (log 0d0) twice on the second time it won't produce an error.
That doesn't happen with (/ 0d0 0d0).
At first it seemed easy, just pry MXCSR from the context and restore it, but it has some bogus value, with all exception bits being set, both in foreign and lisp code.