Fix nearbyintl linkage for ia64 (bug 19219)

GLIBC fails to build for ia64 since commit d0d286d3.
It is because this commit uses the internal definition for nearbyintl,
but ia64 specialized implementation (sysdeps/ia64/fpu/s_nearbyintl.S)
does not define it.

Tested with a ia64 build.

        [BZ #19219]
        * sysdeps/ia64/fpu/s_nearbyintl.S (__nearbyint): Define and
        weak_alias to nearbyintl.