Simplify behavior of code stubs that accept a variable number of stack

Simplify behavior of code stubs that accept a variable number of stack
arguments in addition to their parameters. Before, we'd add a special
variable to the environment with the value of a register with the
number of arguments. Now, that register just appears as a parameter to
the code stub.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/70203002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17680 ce2b1a6d-e550-0410-aec6-3dcde31c8c00