hashtable_policy.h (_Prime_rehash_policy): Use __builtin_floor rather than __builtin_ceil to compute next resize value.

2011-07-24  François Dumont  <francois.cppdevs@free.fr>

        * include/bits/hashtable_policy.h (_Prime_rehash_policy): Use
        __builtin_floor rather than __builtin_ceil to compute next resize
        value.
        * testsuite/23_containers/unordered_set/hash_policy/load_factor.cc:
        New.

From-SVN: r176717