Performing `block_given?` from inside an orphan block always returns false

[dearblue]

I noticed that while working on #5038, when I do a block_given? from within an orphan block, it always returns false.
When I tried the old mruby, it returned true until mruby-1.3.0.

% for i in 1.2.0 1.3.0 1.4.0 1.4.1 2.0.0 2.0.1 2.1.0 2.1.1 \
  do \
    echo -n "mruby-$i  " \
    mruby-$i/build/host/bin/mruby -e 'def f(&b); -> { [!!b, block_given?] }; end; p f{}.call' \
  done
mruby-1.2.0  [true, true]
mruby-1.3.0  [true, true]
mruby-1.4.0  [true, false]
mruby-1.4.1  [true, false]
mruby-2.0.0  [true, false]
mruby-2.0.1  [true, false]
mruby-2.1.0  [true, false]
mruby-2.1.1  [true, false]