This CL adds support for updating code objects. So far code objects
were immutable. Sparkplug makes compilation a very frequent operation
and thus wants to avoid copying the instruction stream from the
AssemblerBuffer into the code object (with more overhead that entails).
The idea is to allocate an "empty" Code object initially, which is
likely large enough to hold the full instruction stream. Then Sparkplug
will compile the given function and write the instruction stream
directly into the code object. After compilation is done Sparkplug trims
the Code to the right size and finishes its initialization.

We use relocation_info to determine whether a Code object is fully
initialized: undefined means that this object is filled by SparkPlug
at the moment. If it's a proper ByteArray, this code object is assumed
to be initialized. Turbofan still fully initializes the Code object
immediately.

Before changing the size of the code object, EnsureSweepingCompleted()
makes sure that the code object's page is swept already. This prevents
that the concurrent sweeper loads the new and smaller object size and
stores that memory in the free list.

NotifyCodeObjectChanged() signals the GC that the code object is now
fully initialized and revisits that object (even if it is black already)
to find and record outgoing references in the instruction stream.

Design doc: https://docs.google.com/document/d/12LHGkRXY1H3IFMBrdxs2vhgtG9bfJTdquQUsX1oPoSE/edit?usp=sharing

Bug: v8:11872
Change-Id: Ie1b95b27842eea5ec7e9d345052585a27d6ea7f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2999087
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75582}