nextafter(+0.0,-0.0)=+0.0, and nextafter(-0.0,+0.0)=-0.0,
  but they should return -0.0 and +0.0, respectively. This is
  a known bug in nextafter(3) on AIX, so skip related tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54003 b2dd03c8-39d4-4d8f-98ff-823fe69b080e