signal.c: check stack overflow by SP

* signal.c (check_stack_overflow): raise SystemStackError if SP
  register and fault address is in the same page or the next, on
  x86 Mac OS X.  [EXPERIMENTAL]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45995 b2dd03c8-39d4-4d8f-98ff-823fe69b080e