MIPS: StubCallInterfaceDescriptor and CallInterfaceDescriptor are unified under a base class InterfaceDescriptor.

Port r22448 (a9e0b0e)

Original commit message:
Handling of the context register had to be massaged to effect the unification. This will make it easier to call hydrogen code stubs directly from crankshaft.

BUG=
R=mvstanton@chromium.org, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/400743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22456 ce2b1a6d-e550-0410-aec6-3dcde31c8c00