This removes the last V8_EMBEDDED_BUILTIN ifdefs from the ia32 macro
assembler. It also enables root-relative accesses in a few more spots
where they are cheap.

Bug: v8:6666
Change-Id: I10c8e69d8a76fccd3d0753dad020cced4d92a4a2
Reviewed-on: https://chromium-review.googlesource.com/c/1299018
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56990}