Optimised float->int cast for Win64

I've been compiling CELT for 64-bit Windows and it all seems to work
fine (with the occasional benign warning message) except for one place,
and that is in "float_cast.h" where there is some inline assembly
language that gets complied for Windows. Since the Microsoft 64-bit
compiler won't allow inline assembly language (and *still* doesn't have
"lrintf").