libc++: avoid hash re-definitions

libc++: avoid hash re-definitions

The definitions of hash<> for int64_t/uint64_t that were not available
on i386 in the __gnu_cxx namespace are available when we switch over to
std::tr1 namespace so we remove them to avoid the redefinition errors.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>