MIPS: Simplify behavior of code stubs that accept a variable number of stack arguments in addition to their parameters.

Port r17680 (cc0b972)

Original commit message:
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=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/70163006

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17710 ce2b1a6d-e550-0410-aec6-3dcde31c8c00