This is a reland of 65070355

Original change's description:
> [ia32,root] Add indirect calls and jumps through virtual register
>
> This adds a temporary mechanism for isolate-independent calls and
> jumps.  The problem was that - as ia32 doesn't have a scratch register
> - Call and Jump cannot call through a register. This CL adds a
> so-called virtual register (= a pointer-sized field) on IsolateData.
>
> The virtual register can be removed once pc-relative calls are
> implemented and all builtins have been embedded.
>
> Bug: v8:6666
> Change-Id: I1f9d8a25643fad0b3919dd813dbe219d20fcc6bc
> Reviewed-on: https://chromium-review.googlesource.com/c/1282991
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#56693}

TBR=sigurds@chromium.org

Bug: v8:6666
Change-Id: If463e68036673c7873d0d5e1a8a01ef31263cbfa
Reviewed-on: https://chromium-review.googlesource.com/c/1283052
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56700}

This reverts commit 65070355.

Reason for revert: https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Linux%20-%20noi18n%20-%20debug/23308

Original change's description:
> [ia32,root] Add indirect calls and jumps through virtual register
> 
> This adds a temporary mechanism for isolate-independent calls and
> jumps.  The problem was that - as ia32 doesn't have a scratch register
> - Call and Jump cannot call through a register. This CL adds a
> so-called virtual register (= a pointer-sized field) on IsolateData.
> 
> The virtual register can be removed once pc-relative calls are
> implemented and all builtins have been embedded.
> 
> Bug: v8:6666
> Change-Id: I1f9d8a25643fad0b3919dd813dbe219d20fcc6bc
> Reviewed-on: https://chromium-review.googlesource.com/c/1282991
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#56693}

TBR=sigurds@chromium.org,jgruber@chromium.org,ishell@chromium.org

Change-Id: I782651140eee3e75d57ad57d614a981e420fdfeb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/c/1283051Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56694}

This adds a temporary mechanism for isolate-independent calls and
jumps.  The problem was that - as ia32 doesn't have a scratch register
- Call and Jump cannot call through a register. This CL adds a
so-called virtual register (= a pointer-sized field) on IsolateData.

The virtual register can be removed once pc-relative calls are
implemented and all builtins have been embedded.

Bug: v8:6666
Change-Id: I1f9d8a25643fad0b3919dd813dbe219d20fcc6bc
Reviewed-on: https://chromium-review.googlesource.com/c/1282991
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56693}

With this CL we finally actually set the root register to the correct
value.  Verification is still preserved by keeping a magic number in
IsolateData.

Bug: v8:6666
Change-Id: I89cb7cb36f977ac677ec33a814a2798baab4cec4
Reviewed-on: https://chromium-review.googlesource.com/c/1278277Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56627}

... containing RootsTable, ExternalReferenceTable, builtins array and
potentially some other data that can be accessed via the RootRegister.

This is a preliminary step before adding support for pointer-compression
friendly heap layout.

Bug: v8:8182
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I2899f657aaff1351a5304afa0b1a4c5ae4cfc31d
Reviewed-on: https://chromium-review.googlesource.com/c/1245426Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56551}