libstdc++: Fix std::string_view for I32LP16 targets

libstdc++: Fix std::string_view for I32LP16 targets

For H8/300 with -msx -mn -mint32 the type of (_M_len - __pos) is int,
because int is wider than size_t so the operands are promoted.

libstdc++-v3/ChangeLog:

	* include/std/string_view (basic_string_view::copy) Use explicit
	template argument for call to std::min<size_t>.
	(basic_string_view::substr): Likewise.