Tweak the std::hash<> specialization for aarch64_features.

Move the specialization into an explicit std namespace to workaround a
bug in older compilers.  GCC 6.4.1 at least fails to compile the previous
version with the following error:

gdb/arch/aarch64.h:48:13: error: specialization of 'template<class _Tp> struct std::hash' in different namespace [-fpermissive]

  struct std::hash<aarch64_features>