[clang][docs] Specify minimum CMake version for cross compiling

[blabdouze]

The instructions found here https://llvm.org/docs/HowToCrossCompileLLVM.html tell us to set the CMake variable CMAKE_LINKER_TYPE which is only supported since CMake 3.29.

On Ubuntu 22.04 provided CMake is version 3.22 making the cross compilation fail when configurating the project :

cmake -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PROJECTS="lld;clang;clang-tools-extra" \
  -DCMAKE_TOOLCHAIN_FILE=$TARGET-clang.cmake \
  -DLLVM_HOST_TRIPLE=$TARGET \
  -DCMAKE_INSTALL_PREFIX=$HOME/clang-$TARGET \
  -S llvm \
  -B build/$TARGET
-- The C compiler identification is Clang 21.1.5
-- The CXX compiler identification is Clang 21.1.5
-- The ASM compiler identification is Clang with GNU-like command-line
-- Found assembler: /usr/bin/clang-21
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Check for working C compiler: /usr/bin/clang-21
-- Check for working C compiler: /usr/bin/clang-21 - broken
CMake Error at /usr/share/cmake-3.22/Modules/CMakeTestCCompiler.cmake:69 (message):
  The C compiler

    "/usr/bin/clang-21"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /home/dev/llvm-project/build/arm-linux-gnueabihf/CMakeFiles/CMakeTmp

    Run Build Command(s):/usr/bin/ninja cmTC_683fc && [1/2] Building C object CMakeFiles/cmTC_683fc.dir/testCCompiler.c.o
    [2/2] Linking C executable cmTC_683fc
    FAILED: cmTC_683fc
    : && /usr/bin/clang-21 --target=arm-linux-gnueabihf --sysroot=/home/dev/sysroot-deb-armhf-stable   CMakeFiles/cmTC_683fc.dir/testCCompiler.c.o -o cmTC_683fc   && :
    /usr/bin/ld: unrecognised emulation mode: armelf_linux_eabi
    Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 elf_iamcu elf_l1om elf_k1om i386pep i386pe
    clang-21: error: linker command failed with exit code 1 (use -v to see invocation)
    ninja: build stopped: subcommand failed.

With CMake 4.1 it works as expected :

 /home/dev/llvm-project/cmake-4.1.2-linux-x86_64/bin/cmake -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PROJECTS="lld;clang;clang-tools-extra" \
  -DCMAKE_TOOLCHAIN_FILE=$PWD/$TARGET-clang.cmake \
  -DLLVM_HOST_TRIPLE=$TARGET \
  -DCMAKE_INSTALL_PREFIX=$HOME/clang-$TARGET \
  -S llvm \
  -B build/$TARGET
-- The C compiler identification is Clang 21.1.5
-- The CXX compiler identification is Clang 21.1.5
-- The ASM compiler identification is Clang with GNU-like command-line
-- Found assembler: /usr/bin/clang-21
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/clang-21 - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++-21 - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- bolt project is disabled
-- clang project is enabled
-- clang-tools-extra project is enabled
-- compiler-rt project is disabled
-- cross-project-tests project is disabled
-- libclc project is disabled
-- lld project is enabled
-- lldb project is disabled
-- mlir project is disabled
-- openmp project is disabled
-- polly project is disabled
-- flang project is disabled
-- libc project is disabled
-- Found Python3: /usr/bin/python3.10 (found suitable version "3.10.12", minimum required is "3.8") found components: Interpreter
...

A line should specify the minimum version of CMake that can be used to cross compile (as done in : https://llvm.org/docs/CMake.html#quick-start)