libstdc++: shrink-to-fit in std::basic_stacktrace::current(skip, max)

If a large stacktrace is reduced to a max depth that is less than half
the capacity it will now be reallocated to remove the unused capacity.

libstdc++-v3/ChangeLog:

	* include/std/stacktrace (basic_stacktrace::current): Reallocate
	a smaller container if the unused capacity is larger than the
	used size.