use shell variable CcLlvmBackend for test
use shell variable CcLlvmBackend for test

Previously we used `AC_DEFINE`d variable `CC_LLVM_BACKEND` which has an empty shell expansion.
sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc