[Ada] GNAT.Sockets: refactor Has_Sockaddr_Len

[Ada] GNAT.Sockets: refactor Has_Sockaddr_Len

Use a field offset computation trick to avoid maintaining a list of
platforms.

2019-07-01  Dmitriy Anisimkov  <anisimko@adacore.com>

gcc/ada/

	* gsocket.h (Has_Sockaddr_Len): Use the offset of sin_family offset in
	the sockaddr_in structure to determine the existence of length field
	before the sin_family.

From-SVN: r272877