configure.in: -mstackrealign

* configure.in: -mstackrealign is necessary for -msse2 working.
  [ruby-core:54716] [Bug #8349]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44539 b2dd03c8-39d4-4d8f-98ff-823fe69b080e