[MLIR] Move from using target_link_libraries to LINK_LIBS for llvm libraries.

When compiling libLLVM.so, add_llvm_library() manipulates the link libraries
being used.  This means that when using add_llvm_library(), we need to pass
the list of libraries to be linked (using the LINK_LIBS keyword) instead of
using the standard target_link_libraries call.  This is preparation for
properly dealing with creating libMLIR.so as well.

Differential Revision: https://reviews.llvm.org/D74864