test_bigmath.rb: ignore on unrelated platforms

* test/bigdecimal/test_bigmath.rb (test_log): ignore the result of log
  which is not a finite number.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44573 b2dd03c8-39d4-4d8f-98ff-823fe69b080e