This CL is the final major step towards shipping off-heap-safe builtins
embedded into the binary.

Prior to snapshot serialization, we now:
* create the embedded blob containing off-heap instruction streams,
* use that to generate embedded.cc (containing embedded binary data),
* replace off-heap-safe builtins with trampolines,
* and serialize those into the final snapshot.

The new RelocInfo::OFF_HEAP_TARGET kind is used to fix up trampoline
targets on deserialization.

Bug: v8:6666
Change-Id: Ib07aea9e3bd7ecdec42291c1388b3a7453ea96ce
Reviewed-on: https://chromium-review.googlesource.com/950775
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51960}