Make arc unit work with ninja builds

  In order to find the llvm-obj directory it has to be (or a soft link
  to it) at one of the following locations:

    ${POLLY_SRC_DIR}/build
    ${POLLY_SRC_DIR}.build
    ${POLLY_SRC_DIR}-build
    s/${POLLY_SRC_DIR}/src/build

  Alternatively, the environment variable $POLLY_BIN_DIR can point to it.

llvm-svn: 244727