configure.in: fix typo

* configure.in (PACKED_STRUCT_UNALIGNED): fix typo.  Intel x86
  cpus are called x86 or i386 etc, not x386.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45172 b2dd03c8-39d4-4d8f-98ff-823fe69b080e