This CL adds support for embedded builtins in nosnap builds by creating
and setting an 'embedded blob' after builtin generation. Unlike
snapshot builds, the blob is not embedded into the .text section but
located on the C++ heap.

This makes nosnap builds more consistent with mksnapshot, and allows us
to simplify there and in serializer cctests.

Complications arise from the different workflows we need to support:

1. the standard mksnapshot build process,
2. nosnap builds (which reuse the blob created by the first Isolate),
2. and tests with various complicated serialization workflows.

To cover all of these cases, this CL introduces two knobs to twiddle:

1. A 'sticky' embedded blob which overrides compiled-in default
   embedded blobs at Isolate setup.
2. The blob lifecycle can be managed manually or through refcounting.

These are described in more detail in isolate.cc.

Tbr: ulan@chromium.org
Bug: v8:6666, v8:8350
Change-Id: I3842e40cdaf45d2cadd05c6eb1ec2f5e3d83568d
Reviewed-on: https://chromium-review.googlesource.com/c/1310195Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57523}