configure.in: use PACKED_STRUCT_UNALIGNED on x64-mingw

* configure.in (PACKED_STRUCT_UNALIGNED): use on x64-mingw too.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45171 b2dd03c8-39d4-4d8f-98ff-823fe69b080e