Show Java stacktrace in JRuby
Show Java stacktrace in JRuby

Java stacktrace includes extra useful information such as locked/waiting
mutex objects (e.g. `- waiting on org.jruby.RubyThread@544fe44c`). This
change shows Java stacktrace in addition to Ruby stacktrace.

`dump_object_count` is disabled because it causes `undefined method`
exception.

frsyuki/sigdump