This is a step towards off-heap (and eventually isolate-independent)
builtins.

Off-heap code cannot use the standard CallStub/CallRuntime mechanisms,
since they directly embed the callee code object pointer within the
caller.  There are two main issues with that: 1. the callee may be
moved by GC, and 2. the pc-relative addressing we currently use breaks
(i.e. ends up pointing to a random spot on the heap) when moving the
caller off-heap.

This CL addresses that by introducing a constants list stored on the
roots array.  Instead of embedding code targets, we now have the option
of loading them from constants list. The code sequence is:

REX.W movq rax,[r13+0x4a0]  // Load the constants cache.
REX.W movq rdx,[rax+0xf]    // From there, load the code target.
...
REX.W addq rdx,0x5f         // Add instruction_start.
call rdx

There's no visible performance impact on the web tooling benchmark.

This list will later be extended to also contain other constants such
as Strings.

Bug: v8:6666
Change-Id: Ifcf67d1f682804ba0b6d3d0383216e16575b6bf5
Reviewed-on: https://chromium-review.googlesource.com/923729
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51434}