Fetch IoT SDK toolchain files

IoT SDK provides some default toolchain files in the `sdk/toolchain.git`
repository. Fetch these files before our top level project is declared
so they can be used when configuring CMake. CMake will only attempt to
find and parse toolchain files when the `project` call is reached. If we
fetch the files before the project is declared, they will be found when
passed to the `--toolchain` option. The `--toolchain` option can be
given a path relative to the build directory.

We place the toolchain files in a `toolchains` folder in the current
build directory. Users can build the SDK using our default toolchains
using either of the following commands.

To build with the arm-none-eabi-gcc toolchain

```
cmake -S . -B __build
--toolchain=toolchains/toolchain-arm-none-eabi-gcc.cmake <iotsdk
configure-options>
```

To build with the armclang toolchain
```
cmake -S . -B __build
--toolchain=toolchains/toolchain-armclang.cmake <iotsdk
configure-options>
```

Signed-off-by: Robert Walton <rob.walton@arm.com>