etc.c: Etc.uname on Windows

* ext/etc/etc.c (etc_uname): add support for Windows using
  GetVersionExW(), GetSystemInfo(), and GetComputerNameExW() with
  `ComputerNameDnsHostname`.  [Feature #9842]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46014 b2dd03c8-39d4-4d8f-98ff-823fe69b080e