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:
```console
$ 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
```