In the near future all ia32 ASM builtins must be audited & possibly refactored
to ensure they do not address ebx (= kRootRegister).

This CL adds mechanisms to verify ebx usage. SupportsRootRegisterScope marks
regions that are root-register-ready (i.e. does not use ebx).
AllowExplicitEbxAccessScope marks regions that are explicitly allowed to use
ebx, e.g. because they spill and restore its value at all boundaries and do not
contain any root-relative accesses.

Consistency is verified by calling the new AssertIsAddressable function at
strategic spots in the Assembler.

All of this code is temporary and should be removed once ia32 fully supports
the kRootRegister.

Bug: v8:6666
Change-Id: I7c5514794db0da889bdae9e3c23bc0d54780879d
Reviewed-on: https://chromium-review.googlesource.com/1226805Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55992}