arm_types: Don't use std::hash<u64>() for LocationDescriptorHash

Apple Clang (clang-600.0.54 on x86_64-apple-darwin13.4.0) complains with:

implicit instantiation of undefined template 'std::__1::hash<unsigned long long>'

src/frontend/arm_types.h

@@ -128,7 +128,7 @@ private:
 
 struct LocationDescriptorHash {
     size_t operator()(const LocationDescriptor& x) const {
-        return std::hash<u64>()(x.UniqueHash());
+        return static_cast<size_t>(x.UniqueHash());
     }
 };