libstdc++: Fix members of net::ip::network_v4

libstdc++-v3/ChangeLog:

	* include/experimental/internet (network_v4::netmask()): Avoid
	undefined shift.
	(network_v4::broadcast()): Optimize and fix for targets with
	uint_least32_t wider than 32 bits.
	(network_v4::to_string(const Allocator&)): Fix for custom
	allocators and optimize using to_chars.
	(operator==(const network_v4&, const network_v4&)): Add missing
	constexpr.
	(operator==(const network_v6&, const network_v6&)): Likewise.
	* testsuite/experimental/net/internet/network/v4/cons.cc: New test.
	* testsuite/experimental/net/internet/network/v4/members.cc: New test.