As a first step towards lazy builtin deserialization, this CL moves
builtins to their own dedicated area in the snapshot blob, physically
located after startup data and before context-specific data.

The startup- and partial serializers now serialize all seen builtins as
references, i.e. they only encode the relevant builtin id (taking care
to preserve special behavior around the interpreter trampoline and
CompileLazy). Builtins are later fully serialized by the
BuiltinSerializer. The separate blobs are finally glued together by
CreateSnapshotBlob.

Deserialization takes the same steps: when we see builtin reference
bytecodes before builtins have been deserialized, we push to a list of
deferred builtin references. After builtin deserialization, this list is
iterated and all builtin references are fixed up.

Bug: v8:6624
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Idee42fa9c92bdbe8d5b8c4b8bf3ca9dd39634004
Reviewed-on: https://chromium-review.googlesource.com/610225Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47596}