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

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

These calculations were changed to use long double nearly ten years ago
in order to get more precision than float:
https://gcc.gnu.org/pipermail/libstdc++/2011-September/036420.html

However, double should be sufficient, whlie being potentially faster
than long double, and not requiring soft FP calculations for targets
without native long double support.

libstdc++-v3/ChangeLog:

	PR libstdc++/96958
	* include/bits/hashtable_policy.h (_Prime_rehash_policy)
	(_Power2_rehash_policy): Use double instead of long double.