Vectorize fp16 tanh and logistic functions on Neon

Activates vectorization of the Eigen::half versions of the tanh and
logistic functions when they run on Neon. Both functions convert their
inputs to float before computing the output, and as a result of this
commit, the conversions and the computation in float are vectorized.

Eigen/src/Core/arch/NEON/PacketMath.h

@@ -4028,6 +4028,7 @@ struct packet_traits<Eigen::half> : default_packet_traits {
     HasCos = 0,
     HasLog = 0,
     HasExp = 0,
+    HasTanh = packet_traits<float>::HasTanh,  // tanh<half> calls tanh<float>
     HasSqrt = 1,
     HasRsqrt = 1,
     HasErf = EIGEN_FAST_MATH,