ext/socket/*.c: trivial struct packing for 64-bit

* ext/socket/ancdata.c ({send,recv}msg_args_struct): 24 => 16 bytes
* ext/socket/init.c (connect_arg): ditto
* ext/socket/raddrinfo.c (getnameinfo_arg): 56 => 48 bytes

Other big stack reductions are less trivial.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47679 b2dd03c8-39d4-4d8f-98ff-823fe69b080e