This introduces further indirections for embedded constants and
external references for builtins generated by the macro-assembler.
The used mechanisms (LookupConstant and LookupExternalReference) are
identical to what we already use in CSA.

Almost all builtins are now isolate-independent in both release and
debug modes. snapshot_blob.bin is roughly 670K smaller in embedded
builds vs. non-embedded builds, while libv8.so is roughly 280K larger.

Bug: v8:6666
Change-Id: I7a6c2193ef5a763e6cf7543dd51597d6fff6c110
Reviewed-on: https://chromium-review.googlesource.com/1006581
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52810}