Add LLVM_DEFAULT_TARGET_TRIPLE to find new location of -lomp

[estewart08]

Summary of proposed changes: An upstream llvm change enables LLVM_ENABLE_PER_TARGET_RUNTIME_DIR by default for the openmp build. This installs the openmp libraries into /opt/rocm-ver/llvm/lib/x86_64-unknown-linux-gnu instead of /opt/rocm-ver/llvm/lib. Currenty, hipBLAS only looks in /lib. Prepend lib/x86_64-unknown-linux-gnu to -L and --rpath to avoid a linker error when the upstream change lands in amd-staging.