Fix problem with float64_bits_string/1 on Windows.

library/float.m:
     Some of the C compilers on Windows (MSVC and some versions of 32-bit
     MinGW GCC) do not support the "ll" size prefix in conversion
     specifiers.  In this case, use an extension to the Microsoft
     C library that provides equivalent functionality.
     (Annoyingly, only the Cygwin GCC raised a warning about this,
     everything else just silently did the wrong thing!)