libstdc++: Value-initialize objects held by EBO helpers [PR 100863]

The allocator, hash function and equality function should all be
value-initialized by the default constructor of an unordered container.
Do it in the EBO helper, so we don't have to get it right in multiple
places.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/100863
	PR libstdc++/65816
	* include/bits/hashtable_policy.h (_Hashtable_ebo_helper):
	Value-initialize subobject.
	* testsuite/23_containers/unordered_map/allocator/default_init.cc:
	Remove XFAIL.
	* testsuite/23_containers/unordered_set/allocator/default_init.cc:
	Remove XFAIL.