[SYCL] Use the system installed vc-intrinsics if available

[sarnex]

For all external dependencies, we should try using the system installed one first and only download the repo if we can't find it.

Apply that for vc-intrinics first, I manually tested this with both static and shared library builds.

[sarnex]

The diff kinda sucks, all I did is wrap all the existing code in

find_package(LLVMGenXIntrinsics QUIET)
if (NOT LLVMGenXIntrinsics_FOUND)

endif()

[aelovikov-intel]

I think it's possible our dev machines could have some obsolete installations. Can we provide an opt-in to download even if there is an installation present?

Or, if the version is important, maybe using the installed an opt-in instead? That would help the folks who requested the change still.

[sarnex]

@aelovikov-intel I don't think we ever installed vc-intrinsics at the system level but sure I can do that

[aelovikov-intel]

@aelovikov-intel I don't think we ever installed vc-intrinsics at the system level but sure I can do that

I expect we'd want to apply the same logic for other dependencies, so even if one of them could have been installed before, it would be a good change.

[sarnex]

@aelovikov-intel We already have two vars to specify custom source/include dirs to use, so I just wrapped the new check to not automatically search if those vars are set.