Make `block_given?` to search for the top of the scope first.
The top of the scope means either:

* the top method body
* the enclosing class body
* the top-level

The special case is the method defined by `define_method` with a
block as in #3841. In cases like this, the method body (given by
a block) is not considered as the top of the scope. You need to use
`&block` in the block parameter if you want to know if a block is
given to the method.

This commit also changes the behavior of `MRB_PROC_SCOPE` flag.
Now it is only set if the `proc` is either a class body or a method
body defined in Ruby. It is no longer set for a block that given to
`define_method`.