This changes Torque's builtin pointers to use a Smi representation
underneath instead of storing the Code target object. Callsites look
up the target entry point through IsolateData::builtin_entry_table.

The notable effect of this CL is that builtin pointer calls no longer
call any on-heap Code.

Bug: v8:7777
Change-Id: Ibf6c749dd46cae7aba51494b09921229dd436f63
Reviewed-on: https://chromium-review.googlesource.com/c/1379880
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58286}