The builtins table is an array of pointers to builtin code objects. It
used to be located within the Builtins class, which itself was part of
the Isolate.

To enable faster isolate-independent access to builtin code objects,
this CL moves the builtins table into the heap, at a constant known
offset from the roots table. With this change, builtins can be accessed
through the root pointer with a single instruction:

 mov reg, [kRootPointer, <offset to builtin>]

TurboAssembler::LookupConstant is also extended in this CL to
potentially shortcut the slow-ish constants table lookup: root
constants are loaded through the root list, and builtin constants
through the builtins table.

Bug: v8:6666
Change-Id: I8e83c2a8783c01ebece89483274f42ab4c8872f2
Reviewed-on: https://chromium-review.googlesource.com/1075275
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53528}