MIPS: Unify JSEntryStub and JSConstructEntryStub, and some more code stub cleanups.

Port r23742 (a68a0c0)

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/548523003

Patch from Balazs Kilvady <balazs.kilvady@imgtec.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23749 ce2b1a6d-e550-0410-aec6-3dcde31c8c00