Ensure that the length property of bound functions are actual unique

Ensure that the length property of bound functions are actual unique
for the individually bound functions.

Our existing code will generate a new function on every call to bind,
but it will use the same shared function. When setting the lenght this
will be set on the shared function, i.e., the length of all bound
functions will be that of the last bound function.
Review URL: http://codereview.chromium.org/7475002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8816 ce2b1a6d-e550-0410-aec6-3dcde31c8c00