[SYCL][libclc] Fix cross builds

[hvdijk]

When performing cross builds, we need native versions of various tools, we cannot assume the cross builds that are part of the current build are executable. LLVM provides the setup_host_tool function to handle this, either picking up versions of tools from LLVM_NATIVE_TOOL_DIR, or implicitly building native versions as needed. Use this in more places.

This applies the changes from LLVM #97392 and #97811 and adapts them to DPC++, and makes the same changes in other places that are only needed for DPC++.

Tested with a suitable cross compilation toolchain file:

$ mkdir build
$ cat >build/aarch64-linux.cmake <<EOF
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR aarch64)

set(CMAKE_C_COMPILER "aarch64-linux-gnu-gcc" CACHE STRING "")
set(CMAKE_CXX_COMPILER "aarch64-linux-gnu-g++" CACHE STRING "")
set(PKG_CONFIG_EXECUTABLE "aarch64-linux-gnu-pkg-config" CACHE STRING "")

set(CMAKE_FIND_ROOT_PATH /usr/aarch64-linux-gnu)
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
EOF
$ python3 buildbot/configure.py -o build/aarch64-linux    \
  --cmake-opt=--toolchain=$PWD/build/aarch64-linux.cmake  \
  --cmake-opt=-DLLVM_HOST_TRIPLE=aarch64-unknown-linux-gnu

[frasercrmck]

This seems to duplicate some work from #16182 - is that right?

Do we know why llvm/llvm-project#97811 and llvm/llvm-project#97392 weren't pulled down correctly? Were those commits just silently dropped?

[hvdijk]

This seems to duplicate some work from #16182 - is that right?

Yes, it looks like this includes the changes from #16182.

Do we know why llvm/llvm-project#97811 and llvm/llvm-project#97392 weren't pulled down correctly? Were those commits just silently dropped?

I suspect the many non-trivial differences between upstream LLVM and DPC++ in libclc's CMake made it easy to miss what did and didn't need to be updated.

[steffenlarsen]

Libclc and runtime-related changes LGTM, but I agree with upstreaming as much as possible. 😉

[hvdijk]

@intel/llvm-gatekeepers This should be good to merge, thanks.

For upstreaming, it looks like upstream LLVM will want these changes only when they also get sycl to make use of it. I think we cannot upstream it just yet, but I will leave llvm/llvm-project#118635 open and others can comment on it if they wish.