Define/guard MLIR_STANDALONE_BUILD LLVM_LIBRARY_OUTPUT_INTDIR var.

It looks like MLIR is using the more modern CMAKE_LIBRARY_OUTPUT_DIRECTORY, but AddLLVM still uses this older LLVM specific alias.

In the specific case I was running into, the empty variable was causing `-Wl,-rpath-link,` on the command line, causing the following argument to be swallowed. This was maddening, because the following argument was the .o file containing `main` and I was getting `main` undefined errors when it was clearly there. This is egregious enough that I chose to guard it.

Differential Revision: https://reviews.llvm.org/D153373