[lanai] Make Lanai backend non-experimental

Summary:
Add Lanai backend to default targets.

Discussion of proposal: http://lists.llvm.org/pipermail/llvm-dev/2016-July/102480.html. ISA added in r279149.

Reviewers: jyknight, rengolin, eliben, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22530

llvm-svn: 279498