Removing Toolchain Submodule

[xmos-jmccarthy]

Thank you for submitting an SDK feature request. Please provide as much information you can.

Describe the feature
It is possible to use cmake to fetch a toolchain file. This must be done before any project is declared.

include(FetchContent)
FetchContent_Declare(xmos_cmake_toolchain
GIT_REPOSITORY https://github.com/xmos/xmos_cmake_toolchain.git
GIT_TAG origin/main
)

FetchContent_MakeAvailable(xmos_cmake_toolchain)

project(...)

Will this change any current APIs? How?
Top level cmake command arg CMAKE_TOOLCHAIN_FILE value would change from xmos_cmake_toolchain/xs3a.cmake to _deps/xmos_cmake_toolchain-src/xs3a.cmake

Who will benefit with this feature?

Any Other info