libstdc++: Fix misleading typedef name in <format>

This local typedef for uintptr_t was accidentally named uint64_t,
probably from a careless code completion shortcut. We don't need the
typedef at all since it's only used once. Just use __UINTPTR_TYPE__
directly instead.

libstdc++-v3/ChangeLog:

	* include/std/format (_Iter_sink<charT, contiguous_iterator>):
	Remove uint64_t local type.

(cherry picked from commit cab0083d)