libstdc++: Use double for unordered container load factors [PR 96958]

libstdc++: Use double for unordered container load factors [PR 96958]

My previous commit for this PR changed the types from long double to
double, but didn't change the uses of __builtin_ceill and
__builtin_floorl. It also failed to change the non-inline functions in
src/c++11/hashtable_c++0x.cc. This should fix it properly now.

libstdc++-v3/ChangeLog:

	PR libstdc++/96958
	* include/bits/hashtable_policy.h (_Prime_rehash_policy)
	(_Power2_rehash_policy): Use ceil and floor instead of ceill and
	floorl.
	* src/c++11/hashtable_c++0x.cc (_Prime_rehash_policy): Likewise.
	Use double instead of long double.