doc: Updated IAR toolchain information for coming version

IAR Arm Toolchain

#################

#. Download and install a release of  `IAR Arm Toolchain`_ (EWARM/CXARM) on your host.

.. note::

   As of now, a Development version of the IAR build tools for Arm is required to work with Zephyr.

   It is distributed to selected partners and customers for evaluation. If you are interested in being

   part of this program, please send a request to the IAR FAE team at fae.emea@iar.com.

#. Download and install a release v9.70 or newer of `IAR Arm Toolchain`_ on your host (IAR Embedded Workbench or IAR Build Tools, perpetual or subscription licensing)

#. Make sure you have :ref:`Zephyr SDK <toolchain_zephyr_sdk>` installed on your host.

.. note::

   A Zephyr SDK is used as a source of tools like device tree compiler (DTC), QEMU, etc… Even though

   IAR Arm toolchain is used for Zephyr RTOS build, still the GNU preprocessor & GNU objcopy might

   be used for some steps like device tree preprocessing and .bin file generation.

#. :ref:`Set these environment variables <env_vars>`:

    - Set :envvar:`ZEPHYR_TOOLCHAIN_VARIANT` to ``iar``.

    - Set :envvar:`IAR_TOOLCHAIN_PATH` to the toolchain installation directory.

#. The IAR Toolchain needs the :envvar:`IAR_LMS_BEARER_TOKEN` environment

   variable to be set to a valid ``license bearer token``.

#. The cloud licensed variant of the IAR Toolchain needs the :envvar:`IAR_LMS_BEARER_TOKEN` environment

   variable to be set to a valid ``license bearer token`` (subscription licensing).

For example:

.. code-block:: bash

    # Linux (default installation path):

      export IAR_TOOLCHAIN_PATH=/opt/iarsystems/bxarm/arm

    export IAR_TOOLCHAIN_PATH=/opt/iar/cxarm-<version>/arm

    export ZEPHYR_TOOLCHAIN_VARIANT=iar

    export IAR_LMS_BEARER_TOKEN="<BEARER-TOKEN>"

.. code-block:: batch

    # Windows:

      set IAR_TOOLCHAIN_PATH=c:\<path>\arm

    set IAR_TOOLCHAIN_PATH=c:\<path>\cxarm-<version>\arm

    set ZEPHYR_TOOLCHAIN_VARIANT=iar

    set IAR_LMS_BEARER_TOKEN="<BEARER-TOKEN>"

.. note::

   The IAR Toolchain uses ``ilink`` for linking. This is incompatible with Zephyr’s

   linker script template, which works with GNU ld. Zephyr’s IAR Arm Toolchain depends on

   Zephyr’s CMake linker script generator, which supports generating icf-files.

   Basic icf-file support is in place, but there are still areas which are not fully

   supported by the CMake linker script generator.

    Known limitations:

.. note::

    - The IAR Toolchain uses ``ilink`` for linking and depends on Zephyr’s CMAKE_LINKER_GENERATOR. ``ilink`` is incompatible with Zephyr’s linker script template, which works with GNU ld.

   The IAR Toolchain uses the GNU Assembler which is distributed with the Zephyr SDK

   for ``.S-files``.

    - The GNU Assembler distributed with the Zephyr SDK is used for ``.S-files``.

.. note::

    - C library support for ``Minimal libc`` only. C++ is not supported.

    - Some Zephyr subsystems or modules may contain C or assembly code that relies on GNU intrinsics and have not yet been updated to work fully with ``iar``.

   Some Zephyr subsystems or modules may also contain C or assembly code that relies

   on GNU intrinsics and have not yet been updated to work fully with ``iar``.

    - TrustedFirmware is not supported

.. _IAR Arm Toolchain: https://www.iar.com/products/architectures/arm/