Reland "[ios][build] Build application extensions in a separate toolchain"

This is a reland of commit 29dfb6fa

There is no change in the reland since the fix is in the internal
repository where the targets will be updated in a separate CL.

Original change's description:
> [ios][build] Build application extensions in a separate toolchain
>
> Add new toolchains to build application extension (widgets, ...)
> and configure the different extensions to use it. This allow to
> disable PA for the extensions and will eventually allow passing
> `-fapplication-extension` to catch at compile time usage of API
> not available in application extensions.
>
> Bug: 1055812
> Fixed: 1449010
> Change-Id: Iacc726e4bc178407094772a2dada3fe8bf08a82a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4580730


> Reviewed-by: Alexis Hétu <sugoi@chromium.org>
> Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
> Reviewed-by: Ernesto Izquierdo Clua <eic@google.com>
> Cr-Commit-Position: refs/heads/main@{#1152640}

Bug: 1055812
Change-Id: I2d23e6aad72a94b10de08704579503cefb2d8f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4588509


Reviewed-by: Ernesto Izquierdo Clua <eic@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1153879}