MIPS: CodeStubs contain their corresponding Isolate* now. (part 1)

Port r20919 (aa51355)

Original commit message:
This is a purely mechanical change, adding an Isolate* to the CodeStub
constructor and a corresponding field plus a getter. A few methods in
CodeStub and its subclasses can be simplified now, but this is done in
a separate CL.

The underlying reason apart from simplicity is that deep down in the
call chain we need to detect if the serializer is active or not. This
information will be part of the Isolate, not a global variable with
funky synchronization primitives around it (which is fundamentally
wrong and the underlying cause for race conditions and a catch-22
during initialization).

BUG=359977
LOG=y
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/252383005

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20944 ce2b1a6d-e550-0410-aec6-3dcde31c8c00