Help GCC compute modiff_incr

Help GCC compute modiff_incr

* src/floatfns.c: Don’t include count-leading-zeros.h,
since we no longer use it directly.
(ecount_leading_zeros): Remove.
(Flogb): Use elogb instead of doing it by hand.
* src/lisp.h: Include count-leading-zeros.h.
(elogb): New macro.
(modiff_incr): Use it so that on typical platforms we
use a hardware instruction instead of a loop.