This is necessary to make sure Polly's dependency on LLVMTarget is properly
registered. Without this change, we got link time failures in bugpoint,
which apparently does not require llvm::TargetRecip::TargetRecip, which however
is needed by Polly ACC.
Registering library dependences for Polly.a will break LLVMPolly.so, hence we
disable the build of LLVMPolly.so when using LINK_POLLY_INTO_TOOLS.