set_backtrace should disable backtrace generation on raise

[headius]

Reported by @jeremyevans while improving perf of no-trace flow control exceptions.

The following benchmarks should perform roughly the same. Instead, the one with set_backtrace performs more like a normal exception with a full backtrace.

3-argument raise:

loop do
  t = Time.now
  1000000.times do
    begin
      raise(Exception, '', [])
    rescue Exception
    end
  puts Time.now - t
end

set_backtrace:

loop do
  t = Time.now
  1000000.times do
    begin
      e = Exception.new
      e.set_backtrace([])
      raise e
    rescue Exception
    end
  puts Time.now - t
end

There's some logic missing in JRuby to avoid generating a backtrace if one is manually set.