targets/opencl: add build-time conflict for LLVM 10.0.0
LLVM 10.0.0 required static linking of Polly otherwise compilation failed. LLVM 10.0.1 statically links Polly to libLLVM which causes issues such as #3494 (closed) due to Polly being present in multiple libraries. As a solution drop static Polly linking and conflict with the only LLVM version which requires it.