Refactored the code for entering and leaving exit frames (calls

Refactored the code for entering and leaving exit frames (calls
from JavaScript to C++). Includes a few slight optimizations
like keeping argv in a callee-saved register.
Review URL: http://codereview.chromium.org/4035

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@359 ce2b1a6d-e550-0410-aec6-3dcde31c8c00