libstdc++: Fix some algos for 16-bit size_t [PR108221]

libstdc++: Fix some algos for 16-bit size_t [PR108221]

Some standard algorithms fail to compile when size_t or ptrdiff_t is
narrower than int. The __lg helper function is ambiguous if ptrdiff_t is
short or __int20, so replace it with a function template that works for
those types as well as signed/unsigned int/long/long long. The helpers
for stable_sort perform arithmetic on size values and assume the types
won't change, which isn't true if the type promotes to int.

libstdc++-v3/ChangeLog:

	PR libstdc++/108221
	* include/bits/stl_algobase.h (__lg): Replace six overloads with
	a single function template for all integer types.
	* include/bits/stl_algo.h (__merge_adaptive_resize): Cast
	arithmetic results back to _Distance.