[UR][CMake] Fix detection of preinstalled OpenCL (#21531)
There were a couple problems with the OpenCL detection.
1) System install detection didn't work because I had to add the
`NO_CMAKE_PACKAGE_REGISTRY` argument to `find_package` to prevent it
finding the `FetchContent` checkout on subsequent calls. This CMake
module is called at least twice, one for `opencl-aot` and once for the
UR OpenCL adapter.
2) If we remove `NO_CMAKE_PACKAGE_REGISTRY` to fix system install
detection, then we hit a problem where if there is a system install, but
it fails the `check_cxx_source_compiles` check because it doesn't
support `cl_khr_spirv_queries`, we had no way to not use the system
install it found, so we can't use `find_package` at all. Use `find_path`
and `find_library`, whcih is what `find_package(OpenCL)` does internally
anyway. Those two functions have nice validator functions we can use
that do exactly what we need, to determine if the found OpenCL install
is suitable, and if not, don't use it. However, this function is only
available with CMake 3.25 (released in 2022) or later, and we currently
require `3.20`, but I think saying we don't support system installs with
older CMake is reasonable, it still fetches and builds OpenCL from
upstream fine.
Also do some minor cleanup like making sure there is always a target
named `OpenCL` that callers can rely on.
Tested the following 3 cases manually:
a) No system OpenCL install
b) System OpenCL install that's too old
c) Working system OpenCL install
and all work as expected.
Closes: https://github.com/intel/llvm/issues/21513
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Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
