ruby.h: use FIX2LONG

* include/ruby/ruby.h (RBIGNUM_SIGN): use FIX2LONG() instead of
  FIX2INT() for optimization, since rb_big_cmp() returns only
  Fixnum -1..+1 for Finxum 0.

  (RBIGNUM_POSITIVE_P, RBIGNUM_NEGATIVE_P): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45001 b2dd03c8-39d4-4d8f-98ff-823fe69b080e