This CL restores consistency with other platforms. ebx is now the
kRootRegister on ia32, and thus not an allocatable register any more.

The PreserveRootIA32 register configuration can be removed since it is
now identical to the Default configuration.

Bug: v8:6666
Change-Id: If4e2b4c52aa8b5db40bcd3b4469004b29c0e3d99
Reviewed-on: https://chromium-review.googlesource.com/c/1299078
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57018}