[polly] [CMake] Link polly-isl-test with LLVMSupport (#65424)

Otherwise link may fail if user provided additional library to link with via CMAKE_EXE_LINKER_FLAGS. Concrete example is using custom allocator, LLVMSupport provides needed -lpthread in that case.

Closes: https://github.com/llvm/llvm-project/pull/65424