[IA64] Use asm-generic/bitops/builtin-ffs.h

The definition of ffs() for ia64 is almost the same as
asm-generic/bitops/builtin-ffs.h.  The only difference is whether it is
defined as inline function or macro function.  So this switches to use
the header (both to reduce amount of arch specific code, and because
inline functions provide type-checking that macros do not).

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>