Added NEON to the list of features required for the aarch64 family

Both floating-point and NEON are required in all standard ARMv8
implementations. Theoretically though specialized markets can support
no NEON or floating-point at all. Source:

https://developer.arm.com/docs/den0024/latest/aarch64-floating-point-and-neon

On the other hand the base procedure call standard for Aarch64
"assumes the availability of the vector registers for passing
floating-point and SIMD arguments". Further "the Arm 64-bit
architecture defines two mandatory register banks: a general-purpose
register bank which can be used for scalar integer processing and
pointer arithmetic; and a SIMD and Floating-Point register bank".
Source:

https://developer.arm.com/docs/ihi0055/latest/procedure-call-standard-for-the-arm-64-bit-architecture

This makes customization of Aarch64 with no NEON instruction set
available so unlikely that we can consider them a feature of the
generic family.

lib/spack/llnl/util/cpu/microarchitectures.json

@@ -904,6 +904,12 @@
       "any_of": [
         "sse4_2"
       ]
+    },
+    "neon": {
+      "reason": "NEON is required in all standard ARMv8 implementations",
+      "families": [
+        "aarch64"
+      ]
     }
   }
 }