configure.in: fix fox BusyBox

* configure.in (rb_cv_binary_elf): get rid of -e option of cat
  which is not available on BusyBox, use tr instead.
  [ruby-core:64824] [Bug #10210]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e