X87: Skip write barriers in the fast case when setting up local context.

Port r21481 (5973b48)

Original commit message:
The FastNewContextStub always allocates in new space, so we don't
need to update the write barrier when copying the parameters to
the newly allocated context.

BUG=
R=danno@chromium.org

Review URL: https://codereview.chromium.org/307433004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21541 ce2b1a6d-e550-0410-aec6-3dcde31c8c00