Add support for Armv8.2-a __fp16

Armv8.2-a provides a native half-precision floating point (__fp16 aka.
float16_t). This patch introduces

* __fp16 as underlying type of Eigen::half if this type is available
* the packet types Packet4hf and Packet8hf representing float16x4_t and
  float16x8_t respectively
* packet-math for the above packets with corresponding scalar type Eigen::half

The packet-math functionality has been implemented by Ashutosh Sharma
<ashutosh.sharma@amperecomputing.com>.

This closes #1940.

Eigen/src/Core/MathFunctions.h

@@ -1846,6 +1846,11 @@ template<> struct random_impl<bool>
   {
     return random<int>(0,1)==0 ? false : true;
   }
+
+  static inline bool run(const bool& a, const bool& b)
+  {
+    return random<int>(a, b)==0 ? false : true;
+  }
 };
 
 template<> struct scalar_fuzzy_impl<bool>