[llvm6] add a compact package (#2263)

* [llvm6] add a compact package

The optional packages including clang, lld, polly, compiler-rt, libcxx, libcxxabi and openmp are included in llvm.

* Update llvm6.rb

* Update llvm6.rb

* Update llvm6.rb

* Update llvm6.rb

* Update llvm6.rb

* Update llvm6.rb

* Update llvm6.rb